the sum of the 15 term of the A.P. 21, 18, 15 ...... is
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a=21
d=-3
Sn=n/2[2a+(n-1)d]
S15=15/2[42+14*-3]
=>7.5[42-42]
=>0
d=-3
Sn=n/2[2a+(n-1)d]
S15=15/2[42+14*-3]
=>7.5[42-42]
=>0
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