Question 17
XYZ is an equilateral triangle, inscribed in a circle. P is a point on the arc YZ such that X and P are on opposite sides of the chord YZ. Which of the following MUST always be true?
Solution
If we join YP and PZ, XYPZ will become a cyclic quadrilateral. YZ and XP will be diagonals of this quadrilateral.
Ptolemy's theorem states that the product of the length of the diagonals is equal to the sum of products of measures of the pairs of opposite sides.
As per this theorem for the quadrilateral :
XY*PZ +XZ*PY = YZ*XP.
For the equilateral triangle.
Now, in equilateral triangle XYZ, YZ = XZ = XY.
Hence equation 1 becomes, XP = YP + PZ.
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