How do you find the third term of (c-d)^8(cd)8?

1 Answer
Apr 3, 2018

color(blue)(28c^6d^2)28c6d2

Explanation:

For binomial expansions of the form (x+y)^n(x+y)n, we have:

sum_(r=0)^(n)((n),(r))x^(n-r)y^r

((n),(r))=color(white)(0)^nC_(r)=(n!)/(r!(n-r)!)

Notice in this form we have the powers of x descending.

We would then expect the power of x in the third term to be:

x^6

So:

x^(8-r)=x^6=>r=2

Using:

((n),(r))

((8),(2))c^(8-2)(-d)^2

((8),(2))=(8!)/(2!(8-2)!)=(8xx7xx6xx5xx4xx3xx2xx1)/(2xx1xx6xx5xx4xx3xx2xx1)

=(8xx7xxcancel(6)xxcancel(5)xxcancel(4)xxcancel(3)xxcancel(2)xxcancel(1))/(2xx1xxcancel(6)xxcancel(5)xxcancel(4)xxcancel(3)xxcancel(2)xxcancel(1))=(8xx7)/(2xx1)=56/2=28

:.

28c^(8-2)(-d)^2=color(blue)(28c^6d^2)