举例说明f(x)=x³+x设x1>x2则，f(x1)-f(x2)=(x1³+x1)-(x2³+x2)=(x1³-x2³)+(x1-x2)=(x1-x2)(x1²+x1*x2+x2²+1)=(x1-x2)[(x1+x2/2)²+3(x2²)/4+1]>0∴ f(x1)>f(x2)∴ f(x)=x³+x在R上单调递增
