Question

Question 5 Find the 4th term in the expansion of (x – 2y)^12 .

Class X1 - Maths -Binomial Theorem Page 171

Answer 1

first of all we have to find out the general term of expansion (x - 2y)¹²

e.g ,
T_{r +1} = nCr.x^(n-r).(-y)^r { general term for (x -y)ⁿ}

here,
n = 12
then,
T_{r+1} = 12Cr.x^{12-r}y^r

for 4th term we have to put r = 3

T_{3+1} = 12C3.x^{12-3}(-2y)^3

= {12!/3!×(12-3)!} x^9(-2y)^3

= -{ 12 × 11 × 10 × /6} x^9 × 8y^3

= -220 × 8x^9y^3

= -1760x^9y^3

hence, 4th term = -1760x^9y^3