图形关于θ=0对称，所以积分区间[0,π]S = 2*1/2*∫(0,π) ρ²dθ =∫(0,π) [2a(2+cosθ)²dθ =4a²∫(0,π) (4+4cosθ+cos²θ)dθ=4a²∫(0,π) (9/2+4cosθ+1/2*cos2θ)dθ=4a²[(9θ/2+4sinθ+1/4*sin2θ](0,π) =18πa²
S = ∫(0 2π) (1/2) ρ^2 dθ = 18πa^2
