the sine function is a many-to-one function and therefore has no inverse function.
However if the domain is restricted to -90° ≤ x ≤ 90°
Then the function is one-to-one for this domain
Thus, 
x is defined as the angle such that - π/2 ≤ x ≤ π/2
5π/6 is therefore outwith the domain
The mean is 7.5 you add up all the numbers and then divide by the amount of numbers there is
Answer:
Well, I'm not sure what you mean but Ptolemy's Theorem gives a relationship between the side lengths and the diagonals of a cyclic quadrilateral; it is the equality case of Ptolemy's Inequality. Ptolemy's Theorem frequently shows up as an intermediate step in problems involving inscribed figures.
