题目内容：

已知△ABC内接于圆O且AB=AC=5cm,∠BAC=120度,则圆O的半径为?

优质解答

BC=(AB^2+AC^2-2*AB*AC*COS(∠BAC))^0.5
=(5^2+5^2-2*5*5*COS(120度))^0.5
=8.6603
2*R=BC/SIN(∠BAC)
=8.6603/SIN(120度)
=10
R=5cm