How do you expand (2x-3)^5 (2x3)5 using Pascal’s Triangle?

1 Answer
Jan 26, 2018

32x^5-240x^4+720x^3-1080x^2+810x-24332x5240x4+720x31080x2+810x243

Explanation:

The binomial theorem states, (x+a)^n=sum_"k=0"^n((n!)/(k!(n-k)!))x^"n-k"a^k(x+a)n=nk=0(n!k!(nk)!)xn-kak

Here, n=5, x=2x,a=-3n=5,x=2x,a=3

(2x-3)^5=sum_"k=0"^5((5!)/(k!(5-k)!))(2x)^"5-k"(-3)^k(2x3)5=5k=0(5!k!(5k)!)(2x)5-k(3)k

When k=0k=0,

((5!)/(0!(5-0)!))(2x)^"5-0"(-3)^0(5!0!(50)!)(2x)5-0(3)0

(120/(1*120))*32x^5*1(1201120)32x51

=32x^5=32x5

When k=1k=1,

((5!)/(1!(5-1)!))(2x)^"5-1"(-3)^1(5!1!(51)!)(2x)5-1(3)1

120/(1*24)*16x^4*(-3)12012416x4(3)

=-240x^4=240x4

When k=2k=2,

((5!)/(2!(5-2)!))(2x)^"5-2"(-3)^2(5!2!(52)!)(2x)5-2(3)2

120/(2*6)*8x^3*9120268x39

=720x^3=720x3

When k=3k=3,

((5!)/(3!(5-3)!))(2x)^"5-3"(-3)^3(5!3!(53)!)(2x)5-3(3)3

120/(6*2)*4x^2*(-27)120624x2(27)

=-1080x^2=1080x2

When k=4k=4,

((5!)/(4!(5-4)!))(2x)^"5-4"(-3)^4(5!4!(54)!)(2x)5-4(3)4

120/(24*1)*2x*811202412x81

=810x=810x

When k=5k=5,

((5!)/(5!(5-5)!))(2x)^"5-5"(-3)^5(5!5!(55)!)(2x)5-5(3)5

120/(120*1)*1*(-243)12012011(243)

=-243=243

Add each individual answer up. The answer is 32x^5-240x^4+720x^3-1080x^2+810x-24332x5240x4+720x31080x2+810x243.