8. find the sum of first 20 terms of an a.p., in which 3rd term is 7 and 7th term is two more than thrice of its 3rd term.
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a3= 7
a7= 3a3 + 2
a7 = 3*7+ 2
a+ 6d = 23
a + 2d = 7
(-) (-) (-)
_____________
4d = 16
d = 4
(By using elimination method )
a+2d = 7
a + 8 = 7
a =(-1)
Sn= n/2(2a + (n-1)d)
= 20/2(2(-1) + (19)(4)
=10(-2+76)
=740
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