Ex 8.2,5

Transcript

Ex 8.2,5 Find the 4th term in the expansion of (x – 2y)12 . We know that General term of expansion (a + b)n is Tr+1 = nCr an–r br We need to find fourth term i.e. T4 = T3+1 of expansion (x – 12y)12 Putting r = 3 , a = x , b = –2y , n = 12 T3+1 = 12C3 (x)12–3 (–2y)3 = ﷐12 !﷮3! (12 −3) !﷯ x9 (–2)3 (y)3 = ﷐12!﷮3! × 9!﷯ x9 (– 8) y3 = ﷐12 × 11 × 10 × 9!﷮3 × 2 × 9 !﷯ × (– 8) × x9 y3 = ﷐12 × 11 × 10 × (−8)﷮6﷯ × x9 y3 = 2 × 11 × 10 × (– 8) × x9 y3 = – 220 × 8 x9 y3 = – 1760 x9 y3