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78,76,74,----------------------- 50 find the sum of terms in that AP |
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Answer» a=78 d=76-78=-2 sn=? sn=n/2(a+l) an=a+(n-1)d 50=78+(n-1)-2 50-78=(n-1)-2 -28=(n-1)-2 -28/-2=(n-1) 14=n-1 n=14+1 n=15 sn=n/2(a+l) sn=15/2(78+50) sn=15/2×128 sn=15×64 sn=960 |
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