y=(x²+3-3)/(x²+3)=1-3/(x²+3)所以y=-3*[-1/(x²+3)²]*(x²+3)'=6x/(x²+3)²
y'=[2x(x²+3)-x²*2x]/(x²+3)² =(6x)/(x²+3)²
y=1-3/(x^2+3)y'=3* [2x/(x^2+3)^2]=6x/(x^2+3)^2
