Statement
Sine squared plus cosine squared is always one:
sin(θ)2+cos(θ)2=1
Proof
Construct a right triangle with one angle being θ, like the image below:

From the definitions from trigonometry follow:
sin(θ)=ba⟹a=bsin(θ)
cos(θ)=bc⟹c=bcos(θ)
Using the Pythagorean theorem, find that:
a2+c2=b2
When substituting a and c, you get:
(bsin(θ))2+(bcos(θ))2=b2
b2sin(θ)2+b2cos(θ)2=b2
Finally, divide each term by b2.
sin(θ)2+cos(θ)2=1