Mathematics

You're finding a perfect square by breaking it into a nearby "round" number. The key insight is to write $91 = 90 + 1$, then use the binomial expansion $(a+b)^2 = a^2 + 2ab + b^2$ where $a = 90$ and $b = 1$. This turns a tricky multiplication into three easy pieces: $90^2$, twice the product $2(90)(1)$, and $1^2$—all mental math.

You're finding a perfect square by breaking it into a nearby "round" number. The key insight is to write $91 = 90 + 1$, then use the binomial expansion $(a+b)^2 = a^2 + 2ab + b^2$ where $a = 90$ and $b = 1$. This turns a tricky multiplication into three easy pieces: $90^2$, twice the product $2(90)(1)$, and $1^2$—all mental math.