The sum of the first ten terms of an A.P. is 160 and the sum of the first two terms of a G.P. is 8. If the first term of the A.P. is equal to the common ratio of the G.P. and the first term of the G.P. is equal to common difference of the A.P., then the sum of all possible values of the first term of the G.P. is :
- 349
- 3413
- 329
- 3213
Answer
(1) 349
Solution
Let the G.P. have first term g and common ratio r
A.P. : first term r, common difference g
A.P. : S10= 102 [2r+9g ]=160 2r+9g=32 … (i)
G.P. : g+gr=8 g(1+r)=8 … (ii)
From (i) : r= 32-9g2 1+r= 34-9g2
Put in (ii) : g 34-9g2=8
9g2-34g+16=0
Discriminant =342-4 9 16=1156-576=580>0
two real values of g, both non-zero (product = 169)
Sum of all possible values of g= 349
Answer: (1)