Proof of quadratic term theorem The coefficients of X in (x + 1) (2x + 1) (3x + 1) (NX + 1) expansion

The two items in each bracket must be multiplied by the other items, and where the product is x, the
x,2x,3x,4x,…… Nx
So the coefficient of X should be 1 + 2 + 3 + n

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