Use the identity
(a + b)² = a ² + 2ab + b ²
Then rewrite 187² as
187² = (100 + 87)² = 100² + 2•100•87 + 87²
Subtract 87² eliminates the last term from above, so we're left with
(100² + 2•100•87 + 87²) - 87²
= 100² + 2•100•87
= 10000 + 2•8700
= 10000 + 17400
= 27400
