Answer
$6a^2 \pi$
Work Step by Step
$\int_{0}^{2 \pi}dy=a\int_{0}^{2 \pi}(2cost-cos2t)(cost-cos2t)dt$
$=2a^2\int_{0}^{2 \pi}[cos2t+1+(\dfrac{cos4t+1}{4})-3(cost)(cos2t)]dt$
$=6a^2 \pi$
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