JEE Advanced · Mathematics · 5. Sequences & Series
Paragraph:
Let \(V_r\) denotes the sum of the first \(r\) terms of an arithmetic progression \((A P)\) whose first term is \(r\) and the common difference is \((2 r-1)\). Let \(T_r=V_{r+1}-V_r-2\) and \(Q_r=T_{r+1}-T_r\) for \(r=1,2, \ldots\)
Question:
\(T_r\) is always
- A an odd number
- B an even number
- C a prime number
- D a composite number
Answer & Solution
Correct Answer
(D) a composite number
Step-by-step Solution
Detailed explanation
\(V_{r+1}-V_r=(r+1)^3-r^3-\frac{1}{2}\) \(\left[(r+1)^2-r^2\right]+\frac{1}{2}(1)\)
\(=3 r^2+2 r+1 \)
\( \therefore T_r =3 r^2+2 r-1=(r+1)(3 r-1)\)
which is a composite number.
\(=3 r^2+2 r+1 \)
\( \therefore T_r =3 r^2+2 r-1=(r+1)(3 r-1)\)
which is a composite number.
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