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Sn= n/2[2a + (n-1)d] 390=n/2 [2(5)+(n-1)3] 390=n/2(10+3n-3) 390=n/2(3n+7) 390=3n^2+7n/2 780=3n^2+7n 3n^2+7n-780=0 (3n+52)(n-15)=0 3n+52=0 3n=-52 ( n should be positive hence -52/3 is not the answer) n-15=0 n=15 (n is equal to 15)
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