4 ms·

I suppose such is possible without digital roots. Let's try the same problem, but this time we'll multiply our constants by 3,571: (x + 14284) (x + 14284)= A:

I suppose such is possible without digital roots. Let's try the same problem, but this time we'll multiply our constants by 3,571: (x + 14284) (x + 14284)= A: x^2 + 204032656x + 28568 B: x^2 + 204032656x + 204032656 C: x^2 + 28568x + 204032656 D: x^2 + 204032656x + 204032656 The digital root of the problem expression is 4. The digital roots of A, B, C and D are 4, 6, 4 and 3 respectively. The digital root of the square of 14284 is 1, and the digital roots of the last term of A and C are 2 and 1, respectively. The answer is C.

dragonwriter 6y ago
Of course, it is quicker just to multiply and add the last digit of the constant to find the only possible answer (or count digits to get an order of magnitude estimate, which will also show C as the only possible answer). When a problem of this form uses big enough numbers to warrant a shortcut, digital root is almost always a suboptimal shortcut.
- jacobwilliamroy 6y ago
  Yes you are the heuristics master!