Dividing through by gives
This can be simplified to:
(\frac{a}{c})^2+(\frac{b}{c})^2=1
Now, a/c is Opposite / Hypotenuse, which is sin(θ)
And b/c is Adjacent / Hypotenuse, which is cos(θ)
So (a/c)2 + (b/c)2 = 1 can also be written:
sin2 θ + cos2 θ = 1