Find the sum of first 24 terms of the A.P. a1, a2, a3, ... if it is known that
a1 + a5 + a10 + a15 + a20 + a24 = 225

• a1 + a5 + a10 + a15 + a20 + a24 = 225
  (a1 + a24)+ (a5 + a20)+ (a10 + a15 ) = 225
  but
  (a1 + a24)= (a5 + a20)= (a10 + a15 )
  hence
  3*(a1+a24) =225
  a1 + a24= 225/3 =75
  
  sum of first 24 terms = (n/2)*(a+l) = 12*75 = 900
  
• 12 years agoHelpfull: Yes(8) No(0)
• We known that sum of 1st term and last term = sum of second term and second last term. Similarly, for all the numbers.
  (a1+a24)+(a5+a20)+(a10+a15)=225
  since, (a1+a24)=(a5+a20)=(a10+a15)
  Therefore, (a1+a24) = 225/3 = 75
  So for 2 terms sum = 75
  Then for 24 terms = ((75*24)/2)=900
• 12 years agoHelpfull: Yes(3) No(0)
• 900 is correct.
• 12 years agoHelpfull: Yes(0) No(3)