How do I find the missing ANGLE of a triangle?

You add the two given angles together. Then you subtract the sum from 180. That gives you the missing angle.

If it's a right angled triangle one angle is 90 degrees, all the internal angles of a triangle add up to 180 degrees, and 90 + 90 = 180 so, the missing angle must combine with the known angle to make 90 degrees.

Call the known angle theta and the unknown angle x.

x + theta = 90 degrees

x = 90 degrees - theta.

Hope this helps.

if you know one angle (say x), the other angle is 90 - x

You can use this to find missing side lengths:
The famous Pythagorean theorem:     a^2 + b^2 = c^2 (c=length of hypotenuse, a,b = other sides)


You can use this to find missing angle or side:
Pythagorean trigonometric identity:  (sohcahtoa)
Sin(Q) = opposite side length/hypotenuse side length
Cos(Q) = adjacent/ hypotenuse
Tan(Q) = opposite/adjacent

where Q =theta = angle

Depending on which side lengths are given you use sine, cosine or tangent.

sin (angle) = (opposite leg) / (hypotenuse)

cos (angle) = (adjacent leg) / (hypotenuse)

tan (angle) = (opposite leg) / (adjacent leg)

If you're looking for an angle and the opposite leg and the hypotenuse are given, then you divide (opposite leg) / (hypotenuse) and get the sin(angle). To find the angle use a calculator to find sin^-1 of the sin(angle). Make sure your calculator is set to "degrees" and not "radians", otherwise you will get the angle in muliples of pi.