dy/dx->k,x->8可得(1/dy)/(1/dx)->k,x->0.于是问题转化为0/0
x->8时f(x)/g(x)=（1/g(x))/(1/f(x))=d(1/g(x))/d(1/f(x))=f(x)*f(x)*g'(x)/(g(x)*g(x)*g'(x))
=>f(x)/g(x)=f'(x)/g'(x)