Question

If the general term is ^91C2 x^89, what is the expansion?

(a) (x)^91

(b) (x – 2)^90

(c) (x – 1)^91

(d) (x + 1)^90

The question was posed to me in an internship interview.

Query is from Binomial Theorem topic in portion Binomial Theorem of Mathematics – Class 11

Answer 1

Correct choice is (c) (x – 1)^91

Easiest explanation: The general term of an expansion is ^nCr x^n – r y^r.

Clearly here n is 91 and the first term is x raised to the power 89.

The second term is raised to power 2.

y^2 = 1

y = +1 or -1

Therefore the expansion can either be (x + 1)^91 or (x – 1)^91.