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समांतर श्रेणी

Question
CBSEHHIMAH10010257

एक A.P. में,
l = 28, S = 144 और कुल 9 पद हैं। a ज्ञात कीजिए।

Solution

यहाँ, l = 28, S = 144
n = 9
l = a + (n – 1)d
28 = a + (9 – 1)d
⇒ 28 = a + 8d ...(i)
Sn space equals space straight n over 2 space left square bracket space 2 straight a space plus space left parenthesis straight n space minus space 1 right parenthesis space straight d space right square bracket
therefore space 144 space equals space 9 over 2 space left square bracket space 2 straight a space plus space left parenthesis 9 space minus space 1 right parenthesis space straight d space right square bracket
rightwards double arrow space 288 over 9 space equals space 2 space left parenthesis space straight a space plus space 4 straight d space right parenthesis
rightwards double arrow 32 = 2( a + 4d )
rightwards double arrow a + 4d = 16         ... (ii)
समीकरण (i) और (ii) को हल करने पर,
a = 32 - 28 = 4
अत:,  a = 4