体积=∫∫D（x²+y²）dxdy=∫∫D（p²）pdpdθ=∫（0,2π）dθ∫（0,√a）p³dp=1/4∫（0,2π）p^4|（0,√a）dθ=1/4∫（0,2π）a²dθ=πa²/2