Trigonometry (Pythagorean Identity)

(sin A)^2 + (cos A)^2 = 1
Square of Height + Square of Base = Square of Slope

Using a Unit Circle with Radius = 1, with a Centre about the Origin (0,0) on the Cartesian Plane, a perpendicular line drawn from any point on the circle's circumference to the X-Axis produces a right angle.

That perpendicular line (height of the triangle) is Opposite to the Angle between the X-Axis and the hypotenuse. It represents the values about the Y-Axis. Since Hypotenuse = 1, sine of angle equals the y values.

The base of the triangle is along the X-Axis and thus represents the values about the X-Axis. It is Adjacent to the Angle. Since Hypotenuse = 1, cosine of angle equals the x values

The slope of the triangle which has a value of 1 which is also the radius of the Unit Circle, is the Hypotenuse. It is opposite to the right angle of the triangle.

Using Pythagoras' Theorem,

Y^2 + X^2  = 1^2
(sin A)^2 + (cos A)^2 = 1