Help with Geometry? (Little trigonometry)?

Trigonometry is based on a picture of a circle with the radius making an angle with the x axis. You carry this picture in your head, not your calculator. Trig is mostly imagination and the rest memorization.

Remember Chief SohCahToa

Sin = Opposite/Hypotenuse

Cosine = Adjacent/Hypotenuse

Tangent = Opposite/Adjacent

Also memorize:

Sec = 1/Cos

Csc = 1/Sin

Cot = 1/Tan

So you draw your unit circle and a 40 degree angle and mark the side O, which is vertical, and the side A, which is horizontal. Then you imagine turning the picture over so the vertical becomes the horizontal. Now you are looking at a 50 degree angle in standard position, but the lines don't change. So the sine of 40 degrees is the same as the cosine of 50 degrees, and in general the sine of any acute angle is identical to the cosine of 90 degrees minus the angle.

sin(x) = O/H = cos(90 - x)

You don't have to memorize this page, but you should remember that there is such a page and you can look it up when you need it: https://en.wikipedia.org/wiki/Trig_identities

And the identities make a lot more sense when you translate them using the O, A, and H as above.