Which of the following is the correct derivative definition?
- \lim_{x\to a}\frac{f\left(x\right)+f\left(a\right)}{x+a}
- \lim_{x\to a}\frac{f\left(x\right)-f\left(a\right)}{x-a}
- \lim_{x\to a}\frac{f\left(x-a\right)-f\left(x\right)}{a}
- \lim_{x\to a}\frac{f\left(a\right)-f\left(x\right)}{a-x}