数学微积分问题.怎么做如下两小题.求助~∫x√(2x+7)dx用带入法：a）u=2x+7b）u^2=2x+7谢谢咯~~~~~~

a) ∫x√(2x+7)dx
= ∫(0.5u-3.5)√u 0.5du
= 0.25[∫u√udu - 7∫√udu]
= 0.25 * 2/5 u^(5/2) - 14/3 u^(3/2) +c
b) x = (u^2-7)/2
dx = udu
∫x√(2x+7)dx
=∫u^2(u^2-7)/2du
.