# Thread: If the radius of the small circle is 8 cm, what is the circumference of the...

1. ## If the radius of the small circle is 8 cm, what is the circumference of the...

I don't know how to do this question, could someone please explain?

2. Hello, iamanoobatmath!

I hope you can follow my labelling.
(I couldn't make a diagram of this one!)

The vertex of the angle is $A.$
Draw a line from $A$ through the centers of the circles.

It first intersects the small circle at $B$,
the center of the smaller circle is $C$,
$D$ is the point of tangency of the two circles,
the center of the larger circle is $E$.

From C, draw a radius down to the point of tangency, $F.$
From E, draw a radius down to the point of tangency, $G.$

We have: . $BC = CD = CF = 8$
Let $DE = EG = r$

In right triangle $CFA,\;\angle CAF = 30^o\,\text{ and }\,CF = 8.$
. . Hence, the hypotenuse $AC = 16.$

Since $\Delta EGA \sim \Delta CFA$, we have: . $\frac{EG}{EA} \:=\:\frac{CF}{CA}$

. . That is: . $\frac{r}{r+24} \:=\:\frac{8}{16} \quad\Rightarrow\quad r \:=\:24$

Therefore, the circumference of the larger circle is: . $48\pi$ cm.