Geometry

Filter
Theory

One of the hardest sections to crack without preparation and one of the easiest with preparation. With so many formulas to learn and remember, this section is going to take a lot of time to master. Remember, read a formula, try to visualize the formula and solve as many questions related to the formula as you can. Knowing a formula and knowing when to apply it are two different abilities. The first will come through reading the formulae list and theory but the latter can come only through solving many different problems.

Theory

Basics of Parallel Lines and their Angles:

  • When two angles A and B are complementary, sum of A and B=90 degrees
  • When two angles A and B are supplementary, sum of A and B=180 degrees
  • When two lines intersect, opposite angles are equal. Adjacent angles are supplementary
  • When any number of lines intersect at a point, the sum of all the angles formed = 360 degrees

Equations of line:

  • General Equation of Line:   Ax + By = C
  • Slope intercept form:   y = mx + c (c is y intercept)
  • Point-slope form:   $$y-y_{1}=m(x-x_{1})$$
  • Intercept form:   $$\frac{x}{a}+\frac{y}{b}=1$$
  • Two-point form:   $$\frac{y-y_{1}}{y_{2}-y_{1}}=\frac{x-x_{1}}{x_{2}-x_{1}}$$

Theory

Consider parallel lines AB, CD and EF as shown in the figure.

Parallel lines with Transversals
  • XY and MN are known as transversals

  • $$\angle $$ XPQ = $$\angle $$ PRS = $$\angle $$ RTU as corresponding angles are equal

  • Interior angles on the side of the transversal are supplementary. i.e. $$\angle $$ PQS + $$\angle $$ QSR = 180 degrees

  • Exterior angles on the same side of the transversal are supplementary. i.e. $$\angle $$ MQB + $$\angle $$ DSU = 180 degrees.

  • Two transversals are cut by three parallel lines in the same ratio i.e. PR / RT = QS / SU

Theory

Triangles

  • The Sum of all angles in a triangle is 180 degrees
  • An angle less than 90 degrees is called an acute angle. An angle greater than 90 degrees is called an Obtuse angle.

  • A triangle with all sides unequal is called a Scalene triangle.

    Nomenclature

  • A line joining the midpoint of a side with the opposite vertex is called a Median.

  • A perpendicular drawn from a vertex to the opposite side is called the Altitude.

  • The three perpendicular bisectors of a triangle meet at a point called the Circumcentre. A circle drawn from this point with the circumradius would pass through all the vertices of the triangle.
  • The three angle bisectors of a triangle meet at a point called the incentre of a triangle. The incentre is equidistant from the three sides and a circle drawn from this point with the inradius would touch all the sides of the triangle.
  • The point of intersection of the three altitudes is the Orthocentre.
  • The point of intersection of the three medians is the Centroid.

  • A triangle with two sides equal is called an Isosceles triangle. The two angles of an isosceles triangle that are not contained between the equal sides are equal.

  • A triangle with all sides equal is called an equilateral triangle. All angles of an equilateral triangle equal 60 degrees.

  • The Sum of any two sides of a triangle is always greater than the third side.
  • The difference between any two sides of a triangle is always lesser than its third side.
  • If in a triangle all of its angles are less than 90^{0} then that triangle is called an Acute-angled triangle.
  • A triangle with one of its angles equal to 90^{0} then that triangle is called a Right-angled triangle.
  • A triangle with one of its angles greater than 90^{0} then that triangle is called an Obtuse angled triangle.
  • If one side of a triangle is produced then that exterior angle formed is equal to the sum of opposite remote interior angles.
  • A line that bisects and also makes a right angle with the same side of the triangle is called a Perpendicular bisector.
    A line that divides the angle at one of the vertices into two parts is called an Angular bisector.
  • Pythagoras theorem: In a right angled triangle ABC where $$\angle $$B=90 degrees, $$ AC^2$$ = $$AB^2$$ + $$BC^2$$
  • Apollonius theorem states that in a triangle ABC if AD is a median to BC then $$AB^2+AC^2=2\cdot\left(AD^2+BD^2\right)$$
Formula

If x is the side of an equilateral triangle then the

Altitude (h) =$$\frac{\sqrt{\ 3}}{2}x$$

Area =$$\frac{\sqrt{\ 3}}{4}x^2$$

Inradius = $$\frac{1}{3}\times\ h$$

Circumradius = $$\frac{2}{3}\times\ h$$

▪ Area of an isosceles triangle =$$\frac{a}{4}\sqrt{\ 4c^2-a^2}$$ (where a, b and c are the length of the sides of BC, AC and AB respectivelyand b = c)

Special triangles :

30^{0}, 60^{0} and 90^{0}

45^{0}, 45^{0} and 90^{0}

Formula

Area of a triangle A

  • $$A$$ = $$\sqrt{s(s-a)(s-b)(s-c)}$$ where s =$$\frac{\left(a+b+c\right)}{2}$$.
  • A =$$\frac{1}{2}\times\ base\times\ altitude$$
  • A = $$\frac{1}{2}\times\ a\times\ b\times\ \sin C$$, where C is the enclosed angle between sides a and b.
  • A = $$\frac{abc}{4R}$$ where R is the circumradius
  • A = rs where r is the inradius and s is the semi perimeter.
Formula

Similarity and Congruence:

Similar triangles :
If two triangles are similar then their corresponding angles are equal and
the corresponding sides will be in proportion.
For any two similar triangles :
▪ Ratio of sides = Ratio of medians = Ratio of heights = Ratio of
circumradii = Ratio of Angular bisectors
▪ Ratio of areas = Ratio of the square of the sides.
Tests of similarity : (AA / SSS / SAS)

Congruent triangles
If two triangles are congruent then their corresponding angles and their
corresponding sides are equal.
Tests of congruence : (SSS / SAS / AAS / ASA)

Theory

Mid-Point Theorem: The line joining the midpoint of any two sides in a triangle is parallel to the third side and is half the length of the third side. If X is the midpoint of CA and Y is the midpoint of CB, then XY will be parallel to AB and XY = ½ * AB

Basic proportionality theorem: If a line is drawn parallel to one side of a triangle and it intersects the other two sides at two distinct points then it divides the two sides in the ratio of respective sides. If in a triangle ABC, D and E are the points lying on AB and BC respectively and DE is parallel to AC then AD/DB = EC/BE

Interior Angular Bisector theorem: In a triangle, the angular bisector of an angle divides the side opposite to the angle, in the ratio of the remaining two sides. In a triangle ABC if AD is the angle bisector of angle A then AD divides the side BC in the same ratio as the other two sides of the triangle. i.e. BD/ CD= AB/AC.

Exterior Angular Bisector theorem: The angular bisector of the exterior angle of a triangle divides the opposite side externally in the ratio of the sides containing the angle. In a triangle ABC, if CE is the angular bisector of exterior
angle BCD of a triangle, then AE/BE = AC/BC

Tip

Important Points to Remember:

  • All points on an angle bisector are equidistant from both arms of the angle.
  • All points on a perpendicular bisector of a line are equidistant from both ends of the line.
  • In an equilateral triangle, the perpendicular bisector, median, angle bisector and altitude drawn from a vertex to a side coincide.
  • If x is the side of an equilateral triangle the altitude is $$\frac{\sqrt{3}}{2}x$$.
  • In a triangle ABC if AD is the angle bisector of angle A then AD bisects the side BC in the same ratio as the other two sides of the triangle. i.e. BD/ CD= AB/AC.
Theory

Circle Properties

  • The angle subtended by the diameter of the circle = 90 degrees
  • Angles subtended by a chord are equal. Also, angles subtended in the major segment are half the angle formed by the chord at the centre
  • The radius from the centre to the point where a tangent touches a circle is perpendicular to the tangent
  • Tangents drawn from the same point to a circle are equal in length
  • A perpendicular drawn from the centre to any chord bisects the chord
  • Equal chords of a circle or equidistant from the centre
  • A polygon of n sides has n(n-3)/2 diagonals
  • In a regular polygon of n sides, each exterior angle is 360/n degrees.
Formula


Area of sector OAXC = $$\frac{\theta\ }{360}\times\pi r^2$$

Area of minor segment AXC = $$\frac{\theta\ }{360}\times\pi r^2-\frac{1}{2}r^2\sin\theta\ $$

Theory

Inscribed angle Theorem : The angle inscribed by the two points lying on the circle, at the centre of the circle is twice the angle inscribed at any point on the circle by the same points:

Angles subtended by the same segment on the circle will be equal. So hereangles a and b will be equal:

Theory
  • Consider a circle as shown in the image. Here, $$AP$$ * $$AQ$$ = $$AS$$ * $$AU$$ = $$AT^2$$
  • Two tangents drawn to a circle from an external common point will be equal in length. So here AZ = AT
  • Two chords that are equal in length will be equidistant from the centre and vice versa
  • When two circles intersect the line joining the centres perpendicularly bisects the line joining the points of intersection
  • The angle made by a chord with a tangent to one of the ends of the chord is equal to the angle subtended by the chord in the other segment. As shown in the figure, angle ACB = angle BAT. Angle tangent chord
Formula

Tangents:

Direct common tangent: $$PQ^2=RS^2=D^2-\left(r_1-r_2\right)^2$$, where D is the distance between the centres:

Transverse common tangent: $$PQ^2=RS^2=D^2-\left(r_1+r_2\right)^2$$, where D is the distance between the centres:

Theory
  • Consider the triangle ABC with incentre I, and the incircle touching the triangle at P,Q,R as shown in the diagram. As tangents drawn from a point are equal, AP=AQ, CP=CR and BQ=BR.
Triangle with Incircle
  • In an equilateral triangle, the centroid divides the median in the ratio 2:1. As the median is also the perpendicular bisector, angle bisector, G is also the circumcentre and incentre.
  • If a is the side of an equilateral triangle, circumradius = $$ a/\sqrt{3}$$ and inradius = $$ = a/2\sqrt{3}$$.
Formula

Area of a quadrilateral

If a quadrilateral has all its vertices on the circle and its opposite angles are supplementary (here x+y = 180^{0}) then that quadrilateral is called a cyclic quadrilateral.

  • In a cyclic quadrilateral, the opposite angles are supplementary
  • Area of a cyclic quadrilateral is $$A$$ = $$\sqrt{(s-a)(s-b)(s-c)(s-d)} $$ where s=(a+b+c+d)/2
  • Exterior angle is equal to its remote interior opposite angle. (here ∠CBX = ∠ADC)
  • A = 1/2 * One diagonal * Sum of offsets drawn to the diagonal
  • For a trapezium, Area = 1/2 * sum of parallel sides * distance between them
  • For a parallelogram, A=Base * Height = Product of two sides * sine of the included angle
  • For a rhombus = 1/2 * Product of diagonals
  • For any solid, Euler's rule states that: Number of faces + Number of vertices = Number of edges +2


Theory

Polygons:

▪ If all sides and all angles are equal, then the polygon is a regular polygon
▪ A regular polygon of n sides has n(n-3)/2 diagonals
▪ In a regular polygon of n sides, each exterior angle is 360/n degrees.
▪ Sum of measure of all the interior angles of a regular polygon is 180 (n-2)
degrees (where n is the number of sides of the polygon)
▪ Sum of measure of all the exterior angles of regular polygon is 360 degrees

Theory


ABCDEF is a regular hexagon with each side equal to ‘x’ then

▪ Each interior angle = 120 degrees

▪ Each exterior angle = 60 degrees

▪ Sum of all the exterior angles = 360 degrees

▪ Sum of all the interior angles = 720 degrees

▪ Area = $$\frac{3\sqrt{\ 3}}{2}a^2$$.

Formula

Characteristics of the four quadrants:

Quadrants


  • Quadrant I: X is Positive, Y is Positive
    Quadrant II: X is Negative, Y is Positive
    Quadrant III: X is Negative, Y is Negative
    Quadrant IV :X is Positive, Y is Negative
    Formula

    TYPE OF SOLIDLATERAL S.A.TOTAL S.A.

    Cube (all sides equal to a)

    4a2

    6a2

    Cuboid (length l, breadth b, height h)

    2(l + b)h

    2(lb + bh + hl)

    Right Circular Cylinder (radius r, height h)

    2πrh

    2πr(h + r)

    Right Circular Cone (radius r, height h, slant height l)

    πrl

    πr(l + r)

    Cone frustum (radii $$r_1$$, $$r_2$$, height h and slant height l)$$\pi(r_1+r_2)l$$$$\pi(r_1+r_2)l+(r_1^2+r_2^2)$$

    Sphere (radius r)

    4πr2

    4πr2

    Solid Hemisphere (radius r)

    2πr2

    3πr2

    Hollow Hemisphere (radius r)

    2πr2

    2πr2

    Pyramid$$\frac{1}{2}\times\ Perimeter\times\ Slant\ Height$$

    $$Curved\ Surface\ area+Base\ area$$

    Prism$$Perimeter\times\ Height$$$$Curved\ Surface\ area+2\times\ Base\ area$$

    Formula

    TYPE OF SOLIDVOLUME

    Cube (all sides equal to a)

    a3

    Cuboid (length l, breadth b, height h)

    lbh

    Right Circular Cylinder (radius r, height h)

    πr2h

    Right Circular Cone (radius r, height h, slant height l)

    $$\frac{1}{3}$$π r2h

    Cone frustum (radii $$r_1$$, $$r_2$$, height h and slant height l)$$V=\frac{1}{3}\pi h(r_2^2+r_1r_2+r_1^2)$$

    Sphere (radius r)

    $$\frac{4}{3}$$πr3

    Solid Hemisphere (radius r)

    $$\frac{2}{3}$$πr3

    Hollow Hemisphere (radius r)

    $$\frac{2}{3}$$πr3

    Pyramid$$\frac{1}{3}\times\ Area\ of\ the\ base\times\ Height$$
    Prism$$Base\ Area\times\ Height$$


    Formula

    Coordinate geometry formulae:

    • The distance between two points with coordinates $$(x_1, y_1), (x_2, y_2)$$ is given by $$ d = \sqrt{(x_2-x_1)^2+(y_2-y_1)^2}$$
    • Mid point between two points $$A\left(x_1,y_1\right)$$ and $$B\left(x_2,y_2\right)$$ is $$\left(\frac{\left(x_1+x_2\right)}{2},\frac{\left(y_1+y_2\right)}{2}\right)$$
    • Coordinates of a point P that divides the line joining $$A\left(x_1,y_1\right)$$ and $$B\left(x_2,y_2\right)$$ internally in the ratio l:m : $$\left(\frac{\left(lx_2+mx_1\right)}{l+m},\frac{\left(ly_2+my_1\right)}{l+m}\right)$$.
    • Coordinates of a point P that divides the line joining $$A\left(x_1,y_1\right)$$ and $$B\left(x_2,y_2\right)$$ externally in the ratio l:m : $$\left(\frac{\left(lx_2-mx_1\right)}{l-m},\frac{\left(ly_2-my_1\right)}{l-m}\right)$$.
    • A line can be defined as $$y=mx+c$$ where m is the slope of the line and c is the y-intercept.
    • Slope $$m=\frac{\left(y_2-y_1\right)}{x_2-x_1}$$. Here, if $$x_2=x_1$$, then the two lines are perpendicular to each other.
    • When two lines are parallel, their slopes are equal i.e $$m_1=m_2$$
    • When two lines are perpendicular, product of their slopes = -1 i.e $$m_1*m_2=-1$$
    • If a and b are the x and y intercept of a line then $$\frac{x}{a}+\frac{y}{b}=1$$
    • If two intersecting lines have slopes $$m_1$$ and $$m_2$$, then the angle between the two lines will be $$\tan\theta\ =\frac{\left(m_1-m_2\right)}{1+m_1m_2}$$.
    • The length of perpendicular from a point $$\left(X_1,Y_1\right)$$ on the line AX+BY+C=0 is $$\frac{\left(AX_1+BY_1+C\right)}{\sqrt{\ A^2+B^2}}$$.
    • The distance between two parallel lines Ax+By+C1 = 0 and Ax+By+C2= 0 is $$\left|\frac{C_1-C_2}{\sqrt{\ A^2+B^2}}\right|$$
    Formula
    • The general equation of a circle is x2+y2+2gx+2fy+c=0. Here, the centre of the circle is (-g,-f); the radius of the circle is $$\sqrt{\ g^2+f^2-c}$$.
    • If the centre of the circle lies on the origin, the equation is $$x^2+y^2=r^2$$, where r is the radius of the circle.
    • For a triangle ABC, A (x1, y1), B (x2, y2), C (x3, y3):
      • Centroid= $$\left(\frac{\left(x_1+x_2+x_3\right)}{3},\frac{y_1+y_2+y_3}{3}\right)$$
      • Incentre= $$\left(\frac{\left(ax_1+bx_2+cx_3\right)}{a+b+c},\frac{ay_1+by_2+cy_3}{a+b+c}\right)$$ ;where a, b and c are the lengths of the BC, AC and AB respectively.

    Boost your Prep!

    Download App