Find the sum of `n` terms of series `(x+y) + (x^2+ xy +y^2)+(x^3+x^2y+xy^2+y^3)+..................`

Explanation

We know sum of GP
`S_n​=(a(r^n−1))/(r-1)`
​ when r>1

Where a= first term, n=nth term and r is common ratio
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Given sequence `(x+y),(x^2+xy+y^2),(x^3+x^2y+xy^2+y^3),.`... to n terms.
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Multiplying and dividing each term by (x−y), we get the given sum as
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...