Binomial theorem double sigma (1 Viewer)

[erucibon]

Are we allowed to use double sigma to find the coefficient in for example, (1+x)^2n = (1+x)^n (1+x)^n or do you have to expand it and evaluate by inspection?

 

[Trebla]

Most common approach is to use the general term of one binomial multiplied by the general term of another, but with a different index variable. This is effectively the “double sigma” approach but without explicitly using that notation.