Derivative from first principles
For f(x) = x^2, [f(x+h)-f(x)]/h = [(x+h)^2-x^2]/h = (2xh+h^2)/h = 2x+h. Taking h->0 gives f'(x)=2x.
- Expand before canceling.
- The quotient is defined for h != 0.
- Take the limit after simplification.
derivatives from definitions and efficient rules
For f(x) = x^2, [f(x+h)-f(x)]/h = [(x+h)^2-x^2]/h = (2xh+h^2)/h = 2x+h. Taking h->0 gives f'(x)=2x.
Pause. Find what matters. Then make your move.