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Find the coefficent of ` x^(4)` in the product `(1+2x)^(4) xx (2-x)^(5).` |
Answer» Using the binomial expansion, we get `(1+2x)^(4) = .^(4) C _(0) + .^(4) C _(1) ( 2x) + .^(4) C _(2) (2x)^(2) + .^(4) C _(3) ( 2x) ^ (3) + .^(4) C _(4) ( 2x) ^ (4)` `=1 + 8x + 24x^(2) + 32x^(3) + 16x^(4)` and `( 2-x) ^ (5) = 2^(5) - . ^ (5) C _(1) (2^(4))x + .^(5) C _ (2)(2^(3))x^(2) - .^(5) C _(3) ( 2^(2))x^(3)` `=.^(5) C _(4) ( 2) x^(4) - .^(5) C _ (5) x ^ (5)` `= 32 - 80x + 80x^(2) - 40x^(3) + 10x^(4) - x^(5)` ` :. (1+2x)^(4) xx (2-x)^(5)` ` =(1+ 8x + 24x^(2) + 32x^(3) + 16x^(4))` `xx ( 32 - 80x + 80x^(2) - 40x^(3) + 10x^(4) - x^(5)).` Sum of the terms containing `x^(4)` in the given product `(1 xx 10x^(4)) + 8x (-40x^(3)) + (24x^(2)) (80x^(2))` ` ( 32 x^(3) ) (-80x) + ( 16x^(4) xx 32 ) ` `= 10^(x^(4)) - 320 x^(4) + 1920x^(4) - 2560x^(4) + 512 x^(2) ` `= (10- 320 + 1920 - 2560 + 512) x^(4) = - 438x^(4)`. Hence , the coefficient of `x^(4)` in the given product is `-438`. |
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