if second and third terms of an a.p are 3 ,5 respectively then the sum of first 20 term
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second term (a+d)=3
third term(a+2d)=5
by elimination
a+d =3
a+2d=5
d=2 , a=1
1,3,5,..........
Sn= n/2[2a+(n-1)d]
S20=20/2[2×1+(20-1)2]
S20=10[2+(19)×2]
S20=10[2+38]
S20=10(40)
S20=400
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