a7 =6 in a AP of 13terms then the value of S13 is
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Step-by-step explanation:
a+6d=6
a=6-6d
s13=13/2(2a+12d)
=13/2(a+6d)×2
=13(a+6d)
13(6-6d+6d)
13(6)
78
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