v = ∫adt = ∫(2 + 6x²)dt = 2x + 2x³ + Cx = 0, v = C = 0, v = 2x + 2x³
a=dv/dt=2+6x²dx/dt=v 易得dv/dx=(2+6x²)/v即 vdv=(2+6x²)dx两边积分可得∫vdv=∫(2+6x²)dx即v²/2=2x+2x³+C (C为常数)代入x=0时v=0,得C=0所以v²=4x+4x³
