divisibility rule for 7?

Question

why is the rule of divisibility of 7 not in math textbooks

Answer 1

Because unfortunately, there really isn't a rule that's as simple, easy to remember, and as fast as the other divisibility tests.

If you're interested though, you can find some divisibility rules for 7 here:
http://en.wikipedia.org/wiki/Divisibility_rules#Divisibility_by_7

Answer 2

. To find out if a number is divisible by 7, double the last digit, then subtract it from the remaining digits of the number. If you get an answer divisible by 7, then the original number is divisible by 7. If you don't know whether the new number is divisible by 7, you apply the rule again. For example, to check whether 616 is divisible by 7, double the last digit (6 x 2 = 12), then subtract the answer from the remaining digits (61 – 12 = 49). Because 49 is divisible by 7, so is 616

Answer 3

It's interesting, but not particularly essential to math. (I assume that the rule is that the decimal result for a non-integer result will include repititions of "142857"