from pascal's pyramid
4th term of 5th order is 10
(coefficient - let's call c)
for (a + b)^n the m-th term is
c[a^(n-m+1)][b^(m-1)]
sample (a + b)^3
1st term : c1(a^3b^0) = c1a^3
2nd term : c2(a^2b^1)
3rd term : c3(a^1b^2)
4th term : c4(a^0b^3) = c4b^3
..notice order of 2 variables
one decreases from max to 0, the other increases from 0 to max AND
sum of orders in each term must equal to the order of the binomial
for (2x + 5)^5 , 4th term is
10[(2x)^(5-4+1)][5^(4-1)]
= 10[(2x)^2][5^3)]
= ...
