(a+b)^3=a^3+3a^2*b+3a*b^2+b^3

the pattern of the coefficient:

1

1 2 1

1 3 3 1

1 4 6 4 1

1 5 10 10 5 1

......

the pattern of the exponents :

the exponent of the a from n , decrease 1 each time to 0

the exponent of the b from 0 , increase 1 each time to n