(u/v)'=(u'*v-u*v')/v²
这里u=x,v=√(x²+1)=(x²+1)^(1/2)
u'=1
v'=1/2*(x²+1)^(1/2-1)*(2x)'
=x/√(x²+1)
所以y'=[1*√(x²+1)-x*x/√(x²+1)]/(x²+1)
=[(x²+1-x²)/√(x²+1)]/(x²+1)
=1/[(x²+1)√(x²+1)]