y = 9x - 1/x
I like to rewrite 1/x as x^-1
y = 9x - x^-1
Taking the derivative
dy/dx = 9 - -1 x^-2
= 9 + 1/x^2
That is the first derivative
Now we do it again
dy^2/dx^2 = d/dx ( 9 + 1/x^2)
= d/dx( 9 + x^-2)
= 0 -2x^-3
=-2/x^3
The second derivative is -2 / x^3
