The sum of first 8 term of an Albus 36 and that of first 15 term of the ap is 465 find the sum of first 25 terms of the ap
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Sn=n/2[2a+(n-1) d]
S₈=4(2a+7d)
36=4(2a+7d)
2a+7d=9(eq.1)
S₁₅=15/2(2a+14d)
465=15(a+7d)
a+7d=31(eq.2)
subtracting eq.2 from eq.1 we get,
a=-22
substituting value in eq.1, we get,
-44+7d=9
7d=53
d=53/7
S₂₅=25/2(-44+ 24*53/7)
=25/2(-44+ 1272/7)
=25/2(964/7)
=12050/7
=1721.42
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