If x = √(19 + 4√15) − √(19 − 4√15), then what is the value of x ?

Explanation

x² = (19 + 4√15) + (19 − 4√15) − 2√(361 − 240) = 38 − 2 × 11 = 16, so x = 4 (positive since the first root is larger).