So i am trying to find the kth term for a series used to approximate (1+x)^n

the series is: 1+ n*x+ ((n(n-1))/2!)x^2 +((n(n-1)(n-2))/!)x^3 + ((n(n-1)(n-2)(n-3))/4!)x^4.....

i have figured out the denominatior but i cant figure out the numerator. any help? thanks.