Find the value of the coefficient of
1/x
in the expansion of (2x −
1/x )^5
.

The solution to the problem is as follows:

the genaral term is 5Cr(-1)^r 2^r x^(5-2r) 
and so need 5-2r = -1 => r = 3 
Coeff is then 5C3 (-1)^3 2^3 = -80

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