Q21 of 25 Page 8

Find the 13th term in the expansion of


The index of, is 7, which is an odd natural number.

So, Middle terms are and

= T4 = T3+1 = 7C3(3)7 – 3
                         = 7C3(3)4(x3)3(-1)(6-1)3
                         = 7C3(81)(x)9 (6)-3(-1)3
                         = =
= T5 = T4 + 1 = 7C4(3)7 – 4
                            = 7C4(3)3 (-1)4(x3)(6)-4
                            = 7C4(27) (x)12(6)-4
                            = (35)(27)(6)-4(x)12
                            = =

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