Let `a_(1),a_(2),a_(3), . . .` be a harmonic progression with `a_(1)=5 – InterviewSolution

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1.	Let `a_(1),a_(2),a_(3), . . .` be a harmonic progression with `a_(1)=5anda_(20)=25`. The least positive integer n for which `a_(n)lt0`, isA. 22B. 23C. 24D. 25
Answer» Correct Answer - D `a_(1),a_(2),a_(3),…..` are in H.P. `rArr1/a_(1),1/a_(2),1/a_(3),`… are in A.P. `rArr1/a_(n)=1/a_(1)+(n-1)dlt0` where `(1/25-5/25)/19=d=((-4)/(19xx25)) ` `rArr1/5+(n-1)((-4)/(19xx25))lt0` or `(4(n-1))/(19xx5)gt1` or `n-1gt(19xx5)/4` or `ngt(19xx5)/4`+1 or `nge25`

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