(4) 令 u = √x, 则 x = u^2， dx = 2udu原式 = 2∫ <1, 2>ln(u^2)du = 4∫ <1, 2>lnudu= 4[ulnu]<1, 2> - 4∫ <1, 2>du= 8ln2 - 4 = 4(2ln2-1)
