You have spent years learning to add fractions: find a common denominator, combine numerators, simplify.
Partial fractions run the movie backwards. Start with one rational expression whose denominator is a product of simpler factors, and rewrite it as a
sum of simpler fractions. Why bother? Because a sum like A/(x − a) + B/(x − b) is far easier to integrate, differentiate, or solve in equations than one big fraction with a multiplied denominator. Rule 1 covers the cleanest case: distinct linear factors in the bottom, nothing repeated, nothing quadratic yet.
⚡ BIG IDEA
Partial fractions turn "one awkward fraction" into "several easy ones." It is the reverse of adding over a common denominator.