What is the measure of the radius of the circle inscribed in a triangle whose sides measure $8$, $15$ and $17$ units?
I can easily understand that it is a right angle triangle because of the given edges. but I don't find any easy formula to find the radius of the circle.
5 Answers
$\begingroup$Hint:
use the fact that the area $A$ (of the triangle) is given by: $A=\frac{pr}{2}$ where $p$ is the perimeter and $r$ the incircle radius. This formula can easily be proved ( divide the triangle in three triangle with a common vertex at $O$) and is valid for a convex polygon..
$\endgroup$ 0 $\begingroup$You can easily calculate the area of the triangle.
Then divide the triangle into three smaller ones: $AOB, BOC, COA$. Notice that their areas are respectively $AB\cdot r/2, BC\cdot r/2, CA\cdot r/2$. Add them up and compare to the area of $ABC$ you calculated earlier.
$\endgroup$ $\begingroup$Spotting that the triangle is right-angled is a great help, but the problem can be done without.
Let the sides of the triangle be $a,b,c$ and define the semi-perimeter $s=\frac {a+b+c}2$ the inradius as $r$ to be found and the area of the triangle as $A$.
Then we have both $A=rs$ and Heron's formula for the area of a triangle given the sides $A=\sqrt {s(s-a)(s-b)(s-c)}$ from which we get $$r=\sqrt{\frac {(s-a)(s-b)(s-c)}{s}}=s\sqrt {\left(1-\frac as\right)\left(1-\frac bs\right)\left(1-\frac cs\right)}$$
$\endgroup$ $\begingroup$Notice, $8^2+15^2=17^2$ hence the triangle is right angled triangle.
area of right triangle $$\Delta=\frac{1}{2}\times 8\times 15=60$$
semi-perimeter of right triangle $$s=\frac{8+15+17}{2}=20$$
Radius of inscribed circle is given as $$r=\frac{\Delta}{s}=\frac{60}{20}=3$$
$\endgroup$ 0 $\begingroup$The nswer is $3$, because one formula for the inradius is given by:
$$\frac{\text{area of triangle}}{\text{semi-perimeter of triangle}}$$
