令t=3x+2,则dt=3dx→dx=1/3·dt
∫cos(3x+2)dx
=∫cost·1/3·dt
=1/3·∫cost dt
=1/3·sint+C
=1/3·sin(3x+2)+C则dt=3dx→dx=1/3·dt???不懂t=3x+2，则dt=d(3x+2)=3dx