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If the nth term of the series 25 + 29 + 33 + 37+ …. And 3 + 4 + 6 + 9 + 13 …. are equal, then what is the value of n?
(a) 11
(b) 12
(c) 13
(d) 14

Answer
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Hint: To solve this question first, we will need to find the nth term of the each given series as we can observe first given series are in A.P. and we know that the nth term of the A.P. is given by an=a+(n1)d where a is the first term of the A.P. and d is the common difference of the A.P. similarly to find the nth term of the second series we can observe that difference of the second series is in A.P. so we can let it’s nth term as an2+bn+c where a, b, c are any 3 arbitrary constants(we know a1,a2anda3 terms of the series so make equations and solve for nth term). So, find the nth term of both series and equate them to find the value of n.

Complete step-by-step solution:
We can observe that the first series is in A.P.,
And we need to find of n for which nth term of the both given series are equal.
First series we have,
25 + 29 + 33 + 37 + …
Here first term(a) = 25 and common difference(d) = 29 – 25 = 4,
Hence for the nth term of the first series we get,
an=25+(n1)×4an=4n+21....(1)
Second series we have,
3 + 4 + 6 + 9 + 13 ….
Here we can observe that the difference of the two consecutive terms is in A.P. i.e. 1, 2, 3, 4, … and we know that in that case we need to let the nth term as an2+bn+c where a,bandc are any three random constants now, and we are given that
a1=a+b+c=3a2=4a+2b+c=4a3=9a+3b+c=6
By solving above three equations, we get
a=12,b=12andc=3.
Hence, We get nth term of the second series as,
an=n2n+62.....(2)
Now by equating equations (1) and (2), we get
4n+21=n2n+62
After cross-multiplying, we have
n2n+6=8n+42n29n36=0(n12)(n+3)=0
Hence we get two values of n as 12 and -3 but as n cannot be negative so n=12.
Hence option (b) is the correct answer.


Note: You need to observe both of the given series carefully in order to solve this problem. And this question involves a lot of calculations so make sure to do that carefully also. And remember whenever difference of the given series is in A.P. then we can let the nth term of the series as an2+bn+c where a,bandcare any three random constants.

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