Question

What is the coefficients of x4 in the expansion of (1+x)4(1+2x)3.

Answer 1

By the binomial theorem,

(1 + x)⁴ = 1 + 4x + 6x² + 4x³ + x⁴

(1 + 2x)³ = 1 + 6x + 12x² + 8x³

In the expansion of (1 + x)⁴ (1 + 2x)³, we get an x⁴ when multiplying

• x⁴ and 1 to get x⁴;

• 4x³ and 6x to get 24x⁴;

• 6x² and 12x² to get 72x⁴; and

• 4x and 8x³ to get 32x⁴.

Add these up:

x⁴ + 24x⁴ + 72x⁴ + 32x⁴ = 129x⁴