Find the sum of first 20 terms of an ap in which 3rd term is 7 and 7th term is 2 more than this of its third term
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3rd term =7
a+2d=7- - - 1
7th term = 2 +a+2d=9
a+6d=9- - -2
on subtracting 2&1 and solving
4d=2
d=1/2
putting value of d in 1
a+2d=7
a+2(1/2)=7
a=7-1
a=6
sum of AP=n/2[2a+(n-1)d]
sum of 20 terms of AP=20/2[2(6)+(20-1)1/2]
=10(12+(19/2))
=10[(24+19)/2]
=5*43
=215
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