Question 1141509
Please help me solve this:

Find the fifth term in the expansion of (ab-1) ^20. Use the term formula.
<pre>BINOMIAL EXPANSION term formula: {{{matrix(1,3, (a + b)^n, "=", " " [n]C[r - 1](a)^(n - r + 1)(b)^(r - 1))}}}, where r = term number, or 5, and n = 20.
We then get: 5<sup>th</sup> term of {{{matrix(1,3, (ab  -  1)^20, "=", " " [20]C[5 - 1](ab)^(20 - 5 + 1)(- 1)^(5 - 1))}}}
             5<sup>th</sup> term of {{{matrix(1,3, (ab  -  1)^20, "=", " " [20]C[4](ab)^16(- 1)^(4))}}}
             5<sup>th</sup> term of {{{matrix(1,3, (ab  -  1)^20, "=", "4,845"(ab)^16(1))}}}
             5<sup>th</sup> term of {{{highlight_green(matrix(1,5, (ab  -  1)^20, "=", "4,845" (ab)^16, or, "4,845"a^16b^16))}}}