⇒ For critical points f’(x) = 0

When f’(x) = 0

We get x = 1 or x = -1

When we plot them on number line as f’(x) is multiplied by –ve sign we get

For x>1 function is decreasing

For x<-1 function is decreasing

But between -1 to 1 function is increasing.

∴ Function is increasing in(-1,1).