જો \(a_{1}, a_{2} \ldots, a_{n}\) એ એક સમાંતર શ્રેણી આપેલ છે કે જેનો સામાન્ય તફાવત પૂર્ણાક હોય અને \(S _{ n }= a _{1}+ a _{2}+\ldots+ a _{ n }\) થાય તથા If \(a_{1}=1, a_{n}=300\) અને \(15 \leq n \leq 50,\)હોય તો \(\left( S _{ n -4}, a _{ n -4}\right)\) ની કિમત મેળવો

A \((2480,249)\)
B \((2490,249)\)
C \((2490,248)\)
D \((2480,248)\)

Verified Solution

Answer & Solution

Correct Answer

(C) \((2490,248)\)

Step-by-step Solution

Detailed explanation

\(\quad a_{n}=a_{1}+(n-1) d\) \(\Rightarrow 300=1+(n-1) d\) \(\Rightarrow \quad(n-1) d=299=13 \times 23\) since, \(n \in[15,50]\) \(\therefore n=24\) and \(d=13\) \(a_{n-4}=a_{20}=1+19 \times 13=248\) \(\Rightarrow a_{n-4}=248\) \(S_{n-4}=\frac{20}{2}\{1+248\}=2490\)

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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