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Arithmetic progression

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Question

The sum of all the integers which are divisible by ‘7’ and lying between 50 and 500 is

Moderate
Solution

sum:56,63,70,….497
56+63+70+…+497 are in A.P.
a=56, d=7
tn=497=56+(n-1) tn=a+n-1d
n=64
S64= 64/2[56+497]=17696  Sn=n22a+n-1d  or n2a+l 
 


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