Suppose, α is the angle between hypotenuse and its adjacent side. Then, the measure of angle α is given by;
α = sin-1 (opposite side of α/hypotenuse)
Where sin-1 represents the sine inverse function.
Let α is an angle equal to Sin-1 (-1)
α = sin-1(-1)
We know that, sin 90 degrees = 1
Therefore, α = sin-1 (-sin 90) = -90 degrees= -∏/2
Sin-1 (-1)= -∏/2