Question

If x − y = 5, xy = 84, find x + y.

Answer 1

Here, x − y = 5, xy = 84,

Let x + y = S,

We know:

(x + y)² = (x − y)² + 4xy

So,

S² = 5² + 4 × 84

S² = 25 + 336

S² = 361

∴ S = `+-sqrt361`

∴ S = ±19

Hence, x + y = ±19