Tangents on a circle

Rarely Tested

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:

Number of common tangents based on position:

  • Circles external to each other → 4 tangents
  • Externally touching → 3 tangents
  • Intersecting → 2 tangents
  • Internally touching → 1 tangent
  • One inside other → 0 tangents
Question 1

Two tangents drawn from a point P and a circle with center O at point Q and R. Point A and B lie on PQ and PR, repectively, such that AB is also a tangent to the same circle. If $$\angle AOB=50^{0}$$, then $$\angle APB$$, in degrees equals

Question 2

The sum of the areas of two circles touching each other internally is 233$$\pi$$ sq cm and the distance between the centers of the two circles is 5 cm. Find the ratio of the radii of the two circles?

Question 3

From a point outside a circle, 1001 lines are randomly drawn such that they either touch the circle or pass through the circle. Which of the following can be a possible remainder when the number of points of intersection of the lines with the circle is divided by 6?

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