<s>[INST] How can I make sense of this basic algebraic manipulation backwards? $$\frac{1}{n^2-1} = \frac12\left(\frac1{n-1}-\frac1{n+1}\right)$$
It is easy for me to understand this algebraic from right to left ←, but I struggle to find a reasonable way to work from left to right. 
Regarding expected difficulty, the level of algebraic manipulated is here meant for a Calculus II course. 
Thanks all.
 [/INST] Write $$R = \frac{1}{n^2 - 1} = \frac{1}{(n - 1)(n + 1)}$$
Now, we wish to decompose $R$ into something with denominator $(n - 1)$ and denominator $(n + 1)$
The standard way to do this is by a method knows as "partial fractions". We assign $R$ to 
$$R = \frac{A}{n - 1} + \frac{B}{n + 1} = \frac{1}{(n - 1)(n + 1)}$$ 
Cross multiplying,
$$R = \frac{A (n + 1) + B (n - 1)}{(n - 1)(n + 1)} = \frac{1}{(n - 1)(n + 1)}
$$
Since the denominators are equal, we can simply equate the numerators and see what values of A, B we need.
$$
A(n + 1) + B(n - 1) = 1 + 0n \\
(A - B) + n(A + B) = 1 + 0n
$$
Clearly, we have two inequalities (by assigining the coefficients of the constant term and $n$ to each other.
$$
A - B = 1 \\
A + B = 0
$$
Substituting $B = -A$ in the first equation,
$$
A - (-A) = 1 \\
2A = 1 \\
A = \frac{1}{2}
$$
Substituting $A = \frac{1}{2}$ in $A + B = 0$ gives $B = \frac{-1}{2}$
Hence, $$R = \frac{\frac{1}{2}}{n - 1} + \frac{\frac{-1}{2}}{n + 1} \\
= \frac{1}{2}\left(\frac{1}{n - 1} - \frac{1}{n + 1} \right)$$
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