The possible values of a are -15, -10, 10, and 15.
You can use modular arithmetic.
Considering the number 100, the tens digit of 7^1993 is 4.
By the Law of Cosines, a_1 = 6 - 2*sqrt(2) and a_2 = 6 + 2*sqrt(2).
Solving the system, we get r = -8 and s = 9.
The answer is -41.
There are 38 primes from 200 to 400.
The length of AB is 10*sqrt(3).
