1.

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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