the second derivative of f(x)=ln(x^2+1)

Answer 1

f' = 2 x /(x^2+1)

f" = [ 2(x^2+1)- 2x(2x) ]/(x^2+1)^2

= [ 2x^2+2 -4 x^2 ]/(x^2+1)^2

2[1-x^2] /(x^2+1)^2

Answer 2

can't we cancel them out

Answer 3

cancel what out? There are no common factors.