ABC is an isosceles triangle inscribed in a circle. If AB = AC = 12 5 and BC = 24 cm then radius of circle is:

Answer: Option B

$$\begin{array}{rl}& R_2=\frac{abc}{4△}\\ & △=\sqrt{S\left(s-a\right)\left(s-b\right)\left(s-c\right)}\end{array}$$
$$=\sqrt{12\left(\sqrt{5}+1\right)\left(12\right)\times 12\times 12\left(\sqrt{5}-1\right)}$$
where a = 12$$\sqrt{5}$$ , b = 12$$\sqrt{5}$$  & c = 24
$$\begin{array}{rl}& S=\frac{a+b+c}{2}\\ & S=\frac{24\sqrt{5}+24}{2}\\ & S=12\left(\sqrt{5}+1\right)\\ & R_2=\frac{12\sqrt{5}\times 12\sqrt{5}\times 24}{4\times 12\times 12\times 2}\\ & R_2=\frac{30}{2}\\ & R_2=15\text{cm}\end{array}$$