The sum of the third and seventh terms of an AP is 6, and their product is 8. What is the sum of the first 20th terms of the AP?
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a+2d +a+6d = 6
2a+8d=6
a+4d = 3
a=3-4d-(1)
(a+2d)(a+6d)=8
put value of a from (1)
(3-2d)(3+2d)=8
9-4d^2 = 8
4d^2 =1
d^2 =1/4
d=1/2
a = 3 -4×1/2
a=1
S20 = 10 (2+19×1/2)
=> 115
2a+8d=6
a+4d = 3
a=3-4d-(1)
(a+2d)(a+6d)=8
put value of a from (1)
(3-2d)(3+2d)=8
9-4d^2 = 8
4d^2 =1
d^2 =1/4
d=1/2
a = 3 -4×1/2
a=1
S20 = 10 (2+19×1/2)
=> 115
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