y=x²√(x²-4)
dy/dx=√(x²-4)*2x+x²*1/[2√(x²-4)]*2x
=2x√(x²-4)+x³/√(x²-4)
=[2x(x²-4)+x³]/√(x²-4)
=x(3x²-8)/√(x²-4)
用对数法最快：
lny=ln[x²√(x²-4)]
lny=lnx²+ln√(x²-4)
lny=2lnx+(1/2)ln(x²-4)
y'/y=2/x+(1/2)*1/(x²-4)*(2x)=2/x+x/(x²-4)
y'=[2/x+x/(x²-4)]x²√(x²-4)