Question

Question #2813a

Answer 1

Cos is short for Cosine which is a trigonometric function

Explanation:

Quote from the google dictionary
the trigonometric function that is equal to the ratio of the side adjacent to an acute angle (in a right-angled triangle) to the hypotenuse.

for example in a 3,4,5 triangle, the 5 is the length of the hypotenuse
the angle between the 3 and the 5 is `53.13^0`
the cosine of that angle is written as `cos 53.13^0` and the value of the cos function is `3/5` or `0.6`

in your example
`R^2 = 625 + 225 - 750 *cos(135^0)`

`cos(135^0) = -0.70711`

`R^2 = 625 + 225 - 750 * -0.70711`

`R^2 = 625 + 225 + 530.3301 = 1380.3301`

`R = 37.15279`

You may find the sites useful
https://www.mathsisfun.com/sine-cosine-tangent.html

https://en.wikipedia.org/wiki/Trigonometric_functions