The divisability rule for 7 is that the difference of doubled last digit of a number and the remaining part of that number is divisible by 7.
E.g. 299'999 → 29'999 - 18 = 29'981 → 2'998 - 2 = 2'996 → 299 - 12 = 287 → 28 - 14 = 14 → 14 mod 7 = 0.
It's a very nasty divisibility rule. The one for 13 works in the same way, but instead of multiplying by 2, you multiply by 4. There are actually a couple of well-known rules for that, but these are the easiest to remember IMO.
If someone didn't learn enough trigonometry in school:
1+tan^2^c = cos^2^c/cos^2^c + sin^2^c/cos^2^c = (cos^2^c + sin^2^c)/cos^2^c = 1/cos^2^c;
sqrt(1/cos^2^c) = |sec c|
And here is for the people who still don't get the joke
The reading of the answer is very similar to the word "sexy", which makes the whole sentence a reference to the song "Sexy and I Know It" by LMFAO.