Question

用极坐标计算积分：∫∫ln（1+x^2+y^2)dxdy,其中D是由圆周x^2+y^2=1与两坐标所围成的位于第一象限内的闭区

我算到∫（2/pai,0)dθ∫(1,0)ln(1+r^2)rdr~之后算不下去了啊~麻烦过程写详细点~谢谢~

Answer 1

答：
∫(0到π/2)dθ∫(0到1)ln(1+r^2)rdr
算不定积分∫rln(1+r^2)dr
=∫1/2ln(1+r^2)d(1+r^2)
=1/2∫ln(1+r^2)d(1+r^2)
∫lnxdx=xlnx-x+C
所以1/2∫ln(1+r^2)d(1+r^2)
=1/2[(1+r^2)ln(1+r^2)-(1+r^2)]+C
则∫(0到π/2)dθ∫(0到1)ln(1+r^2)rdr
=π/2∫(0到1)ln(1+r^2)rdr
=π/2[1/2((1+r^2)ln(1+r^2)-(1+r^2))]|(0到1)
=π/4(2ln2-2-(-1))
=(2ln2-1)π/4