JEE Main & Advanced Mathematics Sequence & Series Question Bank Arithmetic Progression

  • question_answer
     Let \[{{T}_{r}}\] be the \[{{r}^{th}}\] term of an A.P. for \[r=1,\ 2,\ 3,....\]. If for some positive integers \[m,\ n\] we have \[{{T}_{m}}=\frac{1}{n}\] and \[{{T}_{n}}=\frac{1}{m}\], then  equals [IIT 1998]

    A) \[\frac{1}{mn}\]

    B) \[\frac{1}{m}+\frac{1}{n}\]

    C) 1

    D) 0

    Correct Answer: C

    Solution :

    \[{{T}_{m}}=a+(m-1)\,d=\frac{1}{n}\] and  \[{{T}_{n}}=a+(n-1)\,d=\frac{1}{m}\] On solving  \[a=\frac{1}{mn}\] and  \[d=\frac{1}{mn}\] \[\therefore \]\[{{T}_{mn}}=a+(mn-1)\,d=\frac{1}{mn}+(mn-1)\frac{1}{mn}=1\]


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