Tangential Distances


$P$ represents a remote point from a circle, and the lines through $PS$ and $PT$ are tangential to the circle.

Prove that the tangential distances $PS$ and $PT$ are always equal.


Consider the following diagram where $C$ represents the centre of the circle.

As a radius meets a tangent at a right angle, $PCS$ and $PCT$ are right angled triangles.

By the Pythagorean Theorem, $(PS)^2 = (PC)^2 - (CS)^2$ and $(PT)^2 = (PC)^2 - (CT)^2$. But as $CS = CT$ (they are both radii), it follows that $(PS)^2 = (PT)^2 \Rightarrow PS = PT$. Q.E.D.

Problem ID: 351 (17 Apr 2009)     Difficulty: 2 Star

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