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
