Tuesday, September 8, 2015

Finding numbers and Arithmetic progression



Here is the question to which I am referring:




Find three digit numbers that are divisible by 5 as well as 9 and whose consecutive digits are in AP.




My way to find answer:





  • I first arbitrarily chosen three numbers ad, a, a+d in AP where ad is at hundreds place , a is at tens place and a+d is at ones place and here d is the common difference.

  • Then we know for a number to be divisible by 5 it should have 0 or 5 at one's place so I first set
    a+d=0.
    But we also know number is divisible by 9 so ((ad)+a) should be equal to 9 or 18
    2ad=9 or 18.
    Now I solved them simultaneously to get two values of a and d but one value was needed to be discarded.
    Same procedure I used for
    a+d=5

    and finally I got three digit numbers whose consecutive digits are in AP
    Those numbers are 630, 135, 765.



So my question is that is there a way to do this problem by another way which involves more AP concepts as I think this way requires very less use of AP concepts.


Answer



I think you can use the fact that (ad)+a+(a+d)=3a is divided by 9.
From here it is clear that a=3 or a=6.
Now we will use the fact that the number is divided by 5.




If a=6 then d must be 1, and if a=3, then d can be 2 or 3.


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