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((x+y)^3)
Expand (x+y)^3 using the binomial theorem. Use row 3 of Pascal's Triangle: [1, 3, 3, 1].
(x+y)^3 = 1x^3 + 3x^2y + 3xy^2 + 1y^3 = x^3 + 3x^2y + 3xy^2 + y^3.
Each term has a coefficient C(3,k), where k ranges from 0 to 3. The powers of x decrease while powers of y increase.
