1.

Find the sum of 5+8+11+14+.........+89

Answer»

an=a+(n-1)d

89=5+(n-1)3

89=5+3n-3

89=2+3n

n=29

Sn=n/2[2A+(n-1)d]

=29/2[10+84]

=29/2[94]

=29×47

=1363



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