Suma de una progresion aritmetica con n terminos S = n / 2(2a + (n - 1)d), para a?

Se sabe la fórmula del término de lugar k <img src="https://tex.z-dn.net/?f=t_k%3Dt_1%2B%28k-1%29d" />

es decir

<img src="https://tex.z-dn.net/?f=t_1%3Dt_1%5C%5C%20%0At_2%3Dt_1%2Bd%5C%5C%20%0At_3%3Dt_1%2B2d%5C%5C%20%0At_4%3Dt_1%2B3d%5C%5C%20%0A%5Cvdots%5C%5C%0At_n%3Dt_1%2B%28n-1%29d%5C%5C%20" />

Si sumamos

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