A.P + GP \neq A.G.P

Algebra Level 4

Let A = { a 1 , a 2 , , a n } A=\{a_1, a_2, \ldots, a_n\} be a set of the first n n terms of an arithmetic progression. Similarly, let B = { b 1 , b 2 , , b n } B=\{b_1, b_2, \ldots, b_n\} be a set of the first n n terms of a geometric progression.

If a new set C = A + B = { a 1 + b 1 , a 2 + b 2 , , a n + b n } C=A+B=\{a_1+b_1, a_2+b_2, \ldots, a_n+b_n\} and the first four terms of C C are { 0 , 0 , 1 , 0 } , \{0, 0, 1, 0\}, what is the 1 1 th 11^\text{th} term of C ? C?


The answer is 117.

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1 solution

Ronak Agarwal
Nov 20, 2014

Let the first term of A . P . A.P. be a 0 {a}_{0} and the first term of G . P . G.P. be g 0 {g}_{0} and the common difference of the A . P . A.P. be d d and the common ratio be r r

Then we have the four equations as :

a 0 + g 0 = 0 {a}_{0}+{g}_{0}=0 (i)

a 0 + d + g 0 r = 0 {a}_{0}+d+{g}_{0}{r}=0 (ii)

a 0 + 2 d + g 0 r 2 = 1 {a}_{0}+2d+{g}_{0}{r}^{2}=1 (iii)

a 0 + 3 d + g 0 r 3 = 0 {a}_{0}+3d+{g}_{0}{r}^{3}=0 (iv)

Using ( i ) (i) in ( i i ) , ( i v ) (ii),(iv) we get :

a 0 + d = a 0 r 0 {a}_{0}+d={a}_{0}{r}_{0} (v)

a 0 + 3 d = a 0 r 3 {a}_{0}+3d={a}_{0}{r}^{3} (vi)

Multiplying ( v ) (v) by 3 3 and subtracting ( v i ) (vi) from it we get :

2 a 0 = a 0 ( 3 r r 3 ) 2{a}_{0}={a}_{0}(3r-{r}^{3}) (vii)

a 0 = 0 \Rightarrow {a}_{0}=0 or r 3 3 r + 2 = 0 {r}^{3}-3r+2=0

But a 0 0 {a}_{0} \neq 0 since then we won't have all our equations satisfied.

r 3 3 r + 2 = 0 \Rightarrow {r}^{3}-3r+2=0

r = 1 , 2 \Rightarrow r=1 , -2

But r 1 r \neq 1 because there also we can't get all the equations satisfied.

Hence r = 2 r=-2

Solving for others we get :

a 0 = 1 9 , g 0 = 1 9 , d = 1 3 , r = 2 {a}_{0}=\frac{-1}{9} , {g}_{0}=\frac{1}{9} , d=\frac{1}{3} , r=-2

So our eleven term of our set is :

a 0 + 10 d + g 0 r 10 = 1 9 + 10 3 + 1024 9 = 117 {a}_{0}+10d+{g}_{0}{r}^{10}=\frac{-1}{9}+\frac{10}{3}+\frac{1024}{9}=117

Perfect !!

Shubhendra Singh - 6 years, 6 months ago

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Nice question!

Joel Tan - 6 years, 6 months ago

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Thanks @Joel Tan .

Shubhendra Singh - 6 years, 6 months ago

That's exactly what I did +1

Oussama Boussif - 6 years, 6 months ago

Did it the same way :D

Aneesh Kundu - 6 years, 6 months ago

correct me if i'm wrong but there's a trivial solution as well where d=a=g=0 and r=1

Afreen Sheikh - 6 years, 5 months ago

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See the 3 r d 3^{rd} term in that case. It would also be 0 in that case. In fact all terms would be 0

Pranjal Jain - 6 years, 4 months ago

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