Sine squared plus cosine squared is always one

Geometry

Statement

Sine squared plus cosine squared is always one:

sin(θ)2+cos(θ)2=1 \sin(\theta)^2 + \cos(\theta)^2 = 1

Proof

Construct a right triangle with one angle being θ \theta , like the image below:

A right triangle with sides a, b and c

From the definitions from trigonometry follow:

sin(θ)=ab    a=bsin(θ) \sin(\theta) = \frac ab \implies a = b\sin(\theta)

cos(θ)=cb    c=bcos(θ) \cos(\theta) = \frac cb \implies c = b\cos(\theta)

Using the Pythagorean theorem, find that:

a2+c2=b2 a^2 + c^2 = b^2

When substituting a a and c c , you get:

(bsin(θ))2+(bcos(θ))2=b2 \bigg(b\sin(\theta)\bigg)^2 + \bigg(b\cos(\theta)\bigg)^2 = b^2

b2sin(θ)2+b2cos(θ)2=b2 b^2 \sin(\theta)^2 + b^2 \cos(\theta)^2 = b^2

Finally, divide each term by b2 b^2 .

sin(θ)2+cos(θ)2=1 \sin(\theta)^2 + \cos(\theta)^2 = 1