设f(u)是连续函数，区域D：x2+y2≤1，则二重积∫∫Df(√x2+y2)dxdy=()
A.2π∫01f(r2)dr B.2π∫01rf(r)dr C.2π∫01f(r)d. D.4π∫01rf(r)dr正确答案B
设f(u)是连续函数，区域D：x2+y2≤1，则二重积∫∫Df(√x2+y2)dxdy=()
A.2π∫01f(r2)dr B.2π∫01rf(r)dr C.2π∫01f(r)d. D.4π∫01rf(r)dr正确答案B
