1.

Find the sum of first 20 multiples of 7 .

Answer»

Multiples of 7 : .7,14,21............

It's forming an A.p 

S = n/2(2a+n-1)d 

  = 20/2 (2*7 + 19*7) 

  = 10* ( 14 + 133) 

   = 1470 




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