Question

Find the 4th term of the expansion of (2a - b)^7.

a.
-560a^4b^3

c.
560a^4b^3

b.
-560a^3b^4

d.
560a^3b^4

Answer 1

-560a^4 b^3.

Explanation:

The (r + 1)th term of (a + x)^n = nCr a^(n-r) x^r.

So, the 4th term of (2a - b)^7 = 7C3 (2a)(7-3)x^3

= 35*16a^4(-b)^3

= -560a^4 b^3.

Answer 2

-560a^4b^3

Explanation:

Given:

(2a - b)^7

By using Binomial theorem:

128a^7 - 448a^6b + 672a^5b^2 - 560a^4b^3 + 280a^3b^4 - 84a^2b^5 + 14ab^6-b^7

Here the fourth term is -560a^4b^3 !