CAT 2018 Slot 2 - QUANT Question 34

Question 34

Let $$t_{1},t_{2}$$,... be real numbers such that $$t_{1}+t_{2}+…+t_{n} = 2n^{2}+9n+13$$, for every positive integer $$n \geq 2$$. If $$t_{k}=103$$, then k equals


Correct Answer: 24

Solution

It is given that $$t_{1}+t_{2}+…+t_{n} = 2n^{2}+9n+13$$, for every positive integer $$n \geq 2$$. 

We can say that $$t_{1}+t_{2}+…+t_{k} = 2k^{2}+9k+13$$   ... (1) 

Replacing k by (k-1) we can say that 

 $$t_{1}+t_{2}+…+t_{k-1} = 2(k-1)^{2}+9(k-1)+13$$   ... (2)

On subtracting equation (2) from equation (1)

$$\Rightarrow$$ $$t_{k} = 2k^{2}+9k+13 - 2(k-1)^{2}+9(k-1)+13$$

$$\Rightarrow$$ $$103 = 4k+7$$

$$\Rightarrow$$ $$k = 24$$


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