How many terms of the AP21,18,15....must be added to get the sum 0?
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Answer:
Sn=0(given)
by formula
Sn=n/2(2a+(n-1)*d)
0=n/2(2*21+(n-1)*-3)
0=n(42-3n+3)
0=n(45-3n)
0=45n-3n2
3n2=45n
n=15
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