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72(vin, given an=4, d= 2, Sn=-14, find n and a.rt |
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Answer» an=a+(n-1)d=4or, a+(n-1)2=4or, a+2n-2=4or, a+2n=4+2or, a+2n=6or, a=6-2nSn=n/2[2a+(n-1)d]=-14or, n/2[2a+(n-1)2]=-14or, an+n(n-1)=-14or, (6-2n)n+n²-n=-14or, 6n-2n²+n²-n=-14or, -n²+5n=-14or, n²-5n=14or, n²-5n-14=0or, n²-7n+2n-14=0or, n(n-7)+2(n-7)=0or, (n-7)(n+2)=0either, n-7=0or, n=7or, n+2=0or, n=-2∵, n can not be negative ;∴, n=7∴, a=6-(2×7)=6-14=-8∴, n=7 and a=-8 Ans. Hit like if you find it useful |
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