Error when integrating absolute value

[Albert Chan]

\(\large I =\int_0^{3\pi \over 2} \int_0^{3\pi \over 2} |\cos y \; \sin x| dx\, dy
= \int_0^{3\pi \over 2} |\sin x| dx \times \int_0^{3\pi \over 2} |\cos y|dy \)

Since it wrap the sine/cosine with the absolute value function, each quadrant sum to area of 1

\(\large → I = 3 × 3 = 9\)