The sum of the third and the seventh terms of an Ap is 20 .find the sum of 13th term
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a3 +a7=20
a+2d+a+6d=20
a+4d=10
a=10-4d
S13=13/2(2a+12d)
S13=13/2x2(a+6d)
S13=13(10-4d+6d)
S13=130-26d
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