√(x2 + x - 1) - √(x2 + 5)
= (√(x2 + x - 1) - √(x2 + 5))·(√(x2 + x - 1) + √(x2 + 5))/(√(x2 + x - 1) + √(x2 + 5))
= ((x2 + x - 1) - (x2 + 5))/(√(x2 + x - 1) + √(x2 + 5))
= (x - 6)/(√(x2 + x - 1) + √(x2 + 5))
= (1 - 6/x)/(√(1 + 1/x - 1/x2) + √(1 + 5/x2))
= 1/2