This wellknown equation is called a
Pythagorean Identity.
It is true for all values of
θ in the unit circle.
Using this first Pythagorean Identity, two additional Pythagorean Identities can be created.

• Start with this first Pythagorean Identity.
• Divide each term by cos^{2}θ.
• We know and .
• Substitute and simplify.

We now have a second Pythagorean Identity:
It should be noted that there are values of θ for which tangent and secant are undefined.
If we divide by a different value, we can arrive at the third identity:

• Start with this first Pythagorean Identity.
• Divide each term by sin^{2}θ.
• We know and .
• Substitute and simplify.

The third Pythagorean Identity is: