수학 트릭: [✔] x + y [✔] x - y [✔] x * y [✔] x / y [✔] x * 11 [✔] x * x x = 15, 25, 35, 45, 55, 65 .. [✔] x * 5 [✔] x * 9 [✔] x * 4 [✔] x / 5 [✔] 1000 - x [✔] x * y y = 2, 4, 8 [✔] 2 ^ x [✔] x + y y = 91 ~ 109, 191 ~ 209, 291 ~ 309 ..