find the sum of the sequence 5+8+11+.....+128
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a=5
d=3
An= a+(n-1)d
128=5+(n-1)3
n-1=123÷3
n-1=41
n=42
S42=42÷2(5+128)
=21 × 133
=2793
d=3
An= a+(n-1)d
128=5+(n-1)3
n-1=123÷3
n-1=41
n=42
S42=42÷2(5+128)
=21 × 133
=2793
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