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Question
Find the 5th term in the expansion of (x – 2y)13.
Solution
General term is tr+1 = nCr xn-r ar
(x – 2y)13 = (x + (-2y))13
Here x is x, a is (-2y) and n = 13
5th term = t5 = t4+1 = 13C4 x13-4 (-2y)4
= 13C4 x9 24 y4
`= (13xx12xx11xx10)/(4xx3xx2xx1)`× 2 × 2 × 2 × 2× x9y4
= 13 × 11 × 10 × 8x9y4
= 13 × 880x9y4
= 11440x9y4
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