【极限符号省略不写】
原式= x[√(x²+1) -x]
= x[√(x²+1) -x] [√(x²+1) +x] / [√(x²+1) +x]
= x/[√(x²+1) +x]
= 1/ [√(1+1/x) +1]
= 1/2