6 thoughts on “Writer’s Block – 4

    • Shalini,

      We can see a pattern here. 2, 22, 222, 2222 etc are equivalent to 2, 4, 6, 8 etc when divided by 9. So their squares will also be equivalent to 2^2, 4^2, 6^2 and so on when divided by 9. Overall we can say the required answer will be equivalent to that of (2^2 + 4^2 + 6^2 +….98^2) when divided by 9, which is the same as [2^2 * (1^2+2^2+3^2+…+49^2)] or [4 * (49*50*99)/6] when divided by 9. I guess it will come 6 (no access to paper right now so…)


  1. Dear J,
    a. What is the intuition and/or logic behind the statement ” the third order differences are same, so it will be a cubic equation” ? How do I arrive at this? in other questions as well ?
    b. In Question 3 it is stated that n>11 , then how can we eliminate the options by using n<11.
    I have seen you do this trick in an earlier question also on the blog. What is the legitimacy of it, is it foolproof ? If not what do we have to look for so as to be sure that taking a value that violates the condition will not affect the answer?


    • It is based on the same logic as “the third derivative of a cubic polynomial is a constant”. If you think about it, a cubic sequence is a discrete form of a cubic polynomial…so will exhibit similar behaviour. And yes, it is pretty much foolproof 🙂


      • Perfect! I got it regarding the cubic sequence. Thanks a lot =D
        I request you to publish some practice questions as well on the concepts discussed. Like the one’s in the Writer’s block and Unwrite, there are only about 3-4 in each of them. If you could provide only questions, we could solve it as an assignment. I hope you consider it 🙂
        Did you say that the cubic method is foolproof or about the trick in Q3?

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