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If `a_(1),a_(2),a_(3),"......"` be in harmonic progression with `a_(1)=5` and `a_(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 HP. `:.(1)/(a_(1)),(1)/(a_(2)),(1)/(a_(3)),"......."` Let D be the common difference of this AP, then `(1)/(a_(20))=(1)/(a_(1))+(20-1)D` `implies D((1)/(25)-(1)/(5))/(19)=-(4)/(25xx19)` and `(1)/(a_(n))=(1)/(a_(1))+(n-1)D=(1)/(5)-(4(n-1))/(25xx19)=((95-4n+4)/(25xx19))` `=((99-4n)/(25xx19))lt0" " [:.a_(n)lt0]` `implies 99-4nlt0 implies ngt24.75`. |
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