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an=a1.q^(n-1)
a1=1/3
------------
a7=243
a1.q^6=243
q^6=729
q=3 or -3
case 1: q=3
S7 = a1(q^7-1)/(q-1) = (1/3)(2187-1)/(3-1) = 1093/3
case 2: q=-3
S7 = a1(q^7-1)/(q-1) = (1/3)(-2187-1)/(-3-1) = 547/3
ie
S7=1093/3 or 547/3
a1=1/3
------------
a7=243
a1.q^6=243
q^6=729
q=3 or -3
case 1: q=3
S7 = a1(q^7-1)/(q-1) = (1/3)(2187-1)/(3-1) = 1093/3
case 2: q=-3
S7 = a1(q^7-1)/(q-1) = (1/3)(-2187-1)/(-3-1) = 547/3
ie
S7=1093/3 or 547/3
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