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If in an AP (S_{10}=210) and the (10)th term is (39), find (S_{20}).यदि किसी समांतर श्रेढ़ी में (S_{10}=210) और (10)वाँ पद (39) है, तो (S_{20}) ज्ञात कीजिए।
Answer and explanationउत्तर और व्याख्या
Correct answer:सही उत्तर: (820)(820)
From (S_{10}=5(a+a_{10})), (a=3), then (d=4) and (S_{20}=820). When a last-like term is given, the average method helps.(S_{10}=5(a+a_{10})) से (a=3), फिर (d=4) और (S_{20}=820)। अंतिम पद जैसा दिया हो तो औसत विधि काम आती है।
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