Problem

Part I Multiple Choice

6. The enclosed area of the curve $(x^2+y^2)^2=x^2-y^2$ can be written in the integral form as

(A) $2\int_0^{\pi/4}\cos2\theta\,\mathrm{d}\theta$.

(B) $4\int_0^{\pi/4}\cos2\theta\,\mathrm{d}\theta$.

(C) $2\int_0^{\pi/4}\sqrt{\cos2\theta}\,\mathrm{d}\theta$.

(D) $\dfrac{1}{2}\int_0^{\pi/4}\cos2\theta\,\mathrm{d}\theta$.