Let A = { a 1 , a 2 , … , a n } be a set of the first n terms of an arithmetic progression. Similarly, let B = { b 1 , b 2 , … , b n } be a set of the first 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 } and the first four terms of C are { 0 , 0 , 1 , 0 } , what is the 1 1 th term of C ?
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Perfect !!
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Nice question!
That's exactly what I did +1
Did it the same way :D
correct me if i'm wrong but there's a trivial solution as well where d=a=g=0 and r=1
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See the 3 r d term in that case. It would also be 0 in that case. In fact all terms would be 0
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Let the first term of A . P . be a 0 and the first term of G . P . be g 0 and the common difference of the A . P . be d and the common ratio be r
Then we have the four equations as :
a 0 + g 0 = 0 (i)
a 0 + d + g 0 r = 0 (ii)
a 0 + 2 d + g 0 r 2 = 1 (iii)
a 0 + 3 d + g 0 r 3 = 0 (iv)
Using ( i ) in ( i i ) , ( i v ) we get :
a 0 + d = a 0 r 0 (v)
a 0 + 3 d = a 0 r 3 (vi)
Multiplying ( v ) by 3 and subtracting ( v i ) from it we get :
2 a 0 = a 0 ( 3 r − r 3 ) (vii)
⇒ a 0 = 0 or r 3 − 3 r + 2 = 0
But a 0 = 0 since then we won't have all our equations satisfied.
⇒ r 3 − 3 r + 2 = 0
⇒ r = 1 , − 2
But r = 1 because there also we can't get all the equations satisfied.
Hence r = − 2
Solving for others we get :
a 0 = 9 − 1 , g 0 = 9 1 , d = 3 1 , r = − 2
So our eleven term of our set is :
a 0 + 1 0 d + g 0 r 1 0 = 9 − 1 + 3 1 0 + 9 1 0 2 4 = 1 1 7