Question
If the general term is 91C2 x89, what is the expansion?
- (x)91
- (x – 2)90
- (x – 1)91
- (x + 1)90
The correct answer is: (x – 1)91
STEP BY STEP SOLUTION
The general term of an expansion is nCr xn – r yr.
Clearly, here n is 91, and the first term is x raised to the power 89.
The second term is raised to power 2.
y2 = 1
y = +1 or -1
Therefore, the expansion can either be (x + 1)91 or (x – 1)91.
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