Question
Wed February 27, 2013 By: Sahil
 

Find the common difference of an AP whose first term is 5 and the sum of its first four terms is half the sum of the next four terms.

Expert Reply
Thu February 28, 2013
a = 5
Since, sum of first four terms is half the sum of next four terms
i.e. a1 + a2+a3 + a4 = 1/2(a5+a6+a7+a8)
a1 + a2+a3 + a4 = 1/2[(a1 + a2+a3 + a4+a5+a6+a7+a8) - (a1 + a2+a3 + a4)] (Adding and subtracting the sum of first four terms from the RHS)
i.e. S4 = 1/2(S8 - S4)
i.e. 2 S4  = S8 - S4 
3S4  = S8


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