The square on the hypotenuse is equal to the sum of the squares on the other two sides.
a2 = b2 + c2
from which we can derive the following relationships between the trig functions:
sin2(θ) + cos2(θ) = 1
sin2(θ) = 1 + tan2(θ)
cosec2(θ) = 1 + cot2(θ)