To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $x-1$ and $1+x$ is $\left(x-1\right)\left(x+1\right)$. Multiply $\frac{x}{x-1}$ times $\frac{x+1}{x+1}$. Multiply $\frac{1}{1+x}$ times $\frac{x-1}{x-1}$.
Since $\frac{x\left(x+1\right)}{\left(x-1\right)\left(x+1\right)}$ and $\frac{x-1}{\left(x-1\right)\left(x+1\right)}$ have the same denominator, add them by adding their numerators.
To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $x-1$ and $1+x$ is $\left(x-1\right)\left(x+1\right)$. Multiply $\frac{x}{x-1}$ times $\frac{x+1}{x+1}$. Multiply $\frac{1}{1+x}$ times $\frac{x-1}{x-1}$.
Since $\frac{x\left(x+1\right)}{\left(x-1\right)\left(x+1\right)}$ and $\frac{x-1}{\left(x-1\right)\left(x+1\right)}$ have the same denominator, add them by adding their numerators.
Consider $\left(x-1\right)\left(x+1\right)$. Multiplication can be transformed into difference of squares using the rule: $\left(a-b\right)\left(a+b\right)=a^{2}-b^{2}$.
For any two differentiable functions, the derivative of the quotient of two functions is the denominator times the derivative of the numerator minus the numerator times the derivative of the denominator, all divided by the denominator squared.
The derivative of a polynomial is the sum of the derivatives of its terms. The derivative of a constant term is $0$. The derivative of $ax^{n}$ is $nax^{n-1}$.
