JEE Mains · Maths · STD 11 - 8. sequence and series
Let \(S_n\) denote the sum of first \(n\) terms an arithmetic progression. If \(S_{20}=790\) and \(S_{10}=145\), then \(S_{15}-\) \(S_5\) is :
- A \(395\)
- B \(390\)
- C \(405\)
- D \(410\)
Answer & Solution
Correct Answer
(A) \(395\)
Step-by-step Solution
Detailed explanation
\(\mathrm{S}_{20}=\frac{20}{2}[2 \mathrm{a}+19 \mathrm{~d}]=790 \) \( 2 \mathrm{a}+19 \mathrm{~d}=79\) \(.............(1)\) \( \mathrm{~S}_{10}=\frac{10}{2}[2 \mathrm{a}+9 \mathrm{~d}]=145 \) \( 2 \mathrm{a}+9 \mathrm{~d}=29\) \(................(2)\) From \((1)\) and \((2)\) a…
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