f.4 a.maths---second derivates

y = x^n + x^(-n)
dy/dx = n x^(n-1) - n x^(-n-1)
d^2 y/dx^2 = n(n-1) x^(n-2) - n(-n-1) x^(-n-2)
= n(n-1) x^(n-2) + n(n+1) x^(-n-2)