d = (26-10)/8 = 2
a1 = 10 - 3*2 = 4
Sn = ( 2*a1 + (n-1)d ) /2 * n
S7 = ( 2*4 + (7-1)*2 ) /2 * 7 = 70
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d = (26-10)/8 = 2
a1 = 10 - 3*2 = 4
Sn = ( 2*a1 + (n-1)d ) /2 * n
S7 = ( 2*4 + (7-1)*2 ) /2 * 7 = 70