1.

Find the coefficient of `x^4`in the expansion of `(x//2-3//x^2)^(10)dot`A. `(405)/(256)`B. `(504)/(259)`C. `(450)/(263)`D. none of these

Answer» Correct Answer - a
Suppose `x^(4)` occours in `(r +1)^(th)` term.
We have ,
`T_(r+1) = ""^(10)C_(r) ((x)/(2))^(10-r) (-(3)/(x^(2)))^(3r) = ""^(10)C_(r) x^(10-3r)(-3)^(r) 2^(r-10)`
This will contain `x^(4)`,if
`therefore 10 - 3r = 4 rArr 3r = 6 rArr r =2`
So, `x^(4)` occours in 3rd and its coefficient is
` ""^(10)C_(r)xx(-3)^(2) xx2^(2-10) =""^(10)C_(2)xx(3^(2))/(2^(8)) = (5xx9xx3^(2))/(2^(8)) = (405)/(256)` .


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