(x+ 1)5 =x5 + 5C1 x4 .1 + 5C2 x3 .12 + 5C3 x2 .13 + 5C4x1 .14 + 5C5x0 .15 (i)
= x5 + 5x4 + 10x3 + 10x2 + 5x + 1 (x– 1)5 = x5 + 5C1x4 (– 1) + 5C2x3 (– 1)2 + 5C3 x2 (– 1)3 + 5C4x(– 1)4 + 5C5x0 (– 1)5 (ii)
= x5 - 5x4 + 10x3 – 10x2 + 5x – 1
Subtracting (ii) from (i),
we get (x + 1)5 – (x– 1)5 = 2[5x4 + 10x2 + 1]
Put x= √2, we get
(√2 + 1)5 − (√2 − 1)5
2 [5 (√2) 4 + 10 (√2) 2 + 1]
= 2[20 + 20 + 1]
= 2 × 41 = 82