A rangefinder would not use Pythagorean as it would need actual height above your target, but its all trig. It probably uses an internal level to gauge the downward or upward angle that you're using and then the distance along the hypotenuse and uses trig.
Someone on here has previously mentioned it, but if you take the cosign of the angle (angle above or below level) and multiply that by the range shown on the rangefinder, that'll give you the true level distance.
If you're looking down at 15 degrees and your old school rangefinder shows 200 yards, to figure out true distance:
1) Make sure you calculator is in degree mode (many default to radians, which for those of you who may not know, is just a different way to measure angle, instead of 360 degrees in a circle, there are 2xpi = ~6.28 radians in a circle). If your calculator is in the wrong mode, when you type in 15, it may think you're talking 15 radians and not 15 degrees.
2) The equation is quite simple, plug in cos(15 degrees) * 200 yards = 0.9659 * 200 = 193 yards true aim.
