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:
Which one of the following is a correct statement?
- A \(Q_1, Q_2, Q_3, \ldots\) are in AP with common difference 5
- B \(Q_1, Q_2, Q_3, \ldots\) are in AP with common difference 6
- C \(Q_1, Q_2, Q_3, \ldots\) are in AP with common difference 11
- D \(Q_1=Q_2=Q_3=\ldots\)
Answer & Solution
Correct Answer
(B) \(Q_1, Q_2, Q_3, \ldots\) are in AP with common difference 6
Step-by-step Solution
Detailed explanation
Since, \(T_r=3 r^2+2 r-1\)
\(\therefore T_{r+1} =3(r+1)^2+2(r+1)-1 \)
\( \therefore Q_r =T_{r+1}-T_r=3[2 r+1]+2[1] \)
\( \Rightarrow Q_r =6 r+5 \)
\( \Rightarrow Q_{r+1} =6(r+1)+5\)
Common difference \(=Q_{r+1}-Q_r=6\)
\(\therefore T_{r+1} =3(r+1)^2+2(r+1)-1 \)
\( \therefore Q_r =T_{r+1}-T_r=3[2 r+1]+2[1] \)
\( \Rightarrow Q_r =6 r+5 \)
\( \Rightarrow Q_{r+1} =6(r+1)+5\)
Common difference \(=Q_{r+1}-Q_r=6\)
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