# Thread: 9-sided polygon circumscribed around a circle

1. ## 9-sided polygon circumscribed around a circle

if a regular 9-sided polygon was circumscribed around a circle, what is the difference between the areas of the polygons

2. ## Area problem

Hello tomatoes
Originally Posted by tomatoes
if a regular 9-sided polygon was circumscribed around a circle, what is the difference between the areas of the polygons
You don't say what the radius of the circle is (so I'll call it $r$), and I assume that you want the difference between the area of the polygon and the area of the circle.

So, imagine the polygon made up of nine isosceles triangles, with a common vertex at the centre of the circle. Then cut one of these triangles in half by a line from the centre to the point where the side of the polygon touches the circle. This line is $r$ units long, and the angle at the centre of the circle in this triangle is $\frac{360}{18}= 20^o$. From this, you should be able to see that the lines joining the centre of the circle to the vertices of the polygon have length $\frac{r}{\cos20^o}$.

Using the formula for the area of a triangle $\tfrac12bc\sin A$, this gives the area of one of the nine original triangles as $\frac12\Big(\frac{r}{\cos20^o}\Big)^2\sin 40^o$.

Multiply this by $9$, and subtract $\pi r^2$, and you're done. OK?