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J & K CET Engineering J and K - CET Engineering Solved Paper-2005

  • question_answer
    If twice the 11 th term of an AP is equal to 7times its 21st term, then its 25th term is equal to

    A)  \[24\]           

    B)  \[120\]

    C)  \[0\]            

    D)  None of these

    Correct Answer: C

    Solution :

    11th term of an \[AP=a+(11-1)\,d=a+10d\] 21st term of an \[AP=a+20d\] and 25th term of an \[AP=a+24d\] According to the given condition, \[2(a+10d)=7(a+20d)\] \[\Rightarrow \] \[2a+20d=7a+140d\] \[\Rightarrow \] \[-5a=120d\] \[\Rightarrow \] \[a=-\frac{120}{5}\,d=-24d\] \[\therefore \]      25th term \[=-24d+24d=0\]


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