Good graphical math sources?

[yj1214]

I'm starting to learning about drawing circles, waves etc. using OpenGL and it involves some math to draw these stuffs and I want to know if there is any source that explains how these things work. For example, this is code for drawing circle.

void drawCircle(float cx, float cy, float r, float num_segments){
	glBegin(GL_LINE_LOOP);

	for(int i = 0; i < num_segments; i++){
                float theta = 2.0f * PI * i / num_segments;

		float x = r * cosf(theta);
		float y = r * sinf(theta);

		glVertex2f(x + cx, y + cy);
	}

	glEnd();
}

This draw a circle but I want to know how it works. Is there any websites/videos that explains how these stuffs work?

[cire]

https://www.khanacademy.org/math/geometry