Cosine

Cosine means the complement to sine. Co-sine. Complement-sine. It is called this because the cosine of an angle is equal to the sine of its complementary angle. Cosine is often written as Cos.

The cosine of an angle is the ratio of the length of the adjacent side divided by the length of the hypotenuse.

Let’s say that the size of the red angle is θ then cos θ = adjacent ÷ hypotenuse

If we use our usual notation and call the adjacent A and the hypotenuse H, then

Cos θ = A ÷ H

So, the cosine of the red angle is adjacent ÷ hypotenuse

The sine of the green angle is opposite ÷ hypotenuse

These are the same as you can see.

The cosine of an angle is equal to the sine of its complementary angle.

Tables of cosines exist just as tables of sines exist. You can find the cosine of angles using the cos button on your calculator in the same way as using the sin button.