What is cis in complex numbers - the exact equation for

Question

1) The number of real roots of of the equation (x-1)² + (x-2)² + (x-3)²=0

2) What is cis in complex numbers?

3) The value of sin (2sin^-1(0.8)) is

4) The exact equation for short distance between two points. 

5) y = (sin^-1(x))³ + (cos^-1(x))³ = (arcsin³(x)) + (arccos³(x))