证：令x1>x2(x1和x2是在定义域上x的两个值）f(x1)-f(x2)=x1+根号下(x1^2+1)-x2+根号下(x2^2+1)=（x1-x2）+根号下(x1^2+1)-根号下(x2^2+1)因为x1>x2所以x1^2>x2^2所以(x1^2+1)>(x2^2+1)所以根号下(x1^2+1)>根号下(x2^...