# What is the answer for question 14 regarding the connection between Pythagorean Trigonometric identities and the Pythagorean Theorem? http://postimg.org/image/dbf27zv7z/

durbanville | High School Teacher | (Level 2) Educator Emeritus

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The following identities show the relationship between the trigonometric functions of a particular angle

`sin^2 theta + cos^2 theta = 1`

`tan^2 theta +1=sec^2 theta`  and

`1+cot ^2 theta = csc ^2 theta`

The reason why they are called the Pythagorean Identities when the Pythagorean Theorem concerns the sides of a right triangle is because they are based on the theorem.

Mathematically, the theorem states `a^2 +b^2=c^2` where c is the hypotenuse in a right-triangle and / or `x^2+y^2=r^2` where r is the radius.

The radius (r)of a unit circle always equals 1 and is used to simplify trigonometric equations and expressions. As r=1, using the standard ratios for sin and cos,which are

`sin theta = (opp)/(hyp)`    and `cos theta = (adj)/(hyp)`

we understand that `sin theta= y/1=y`  and `cos theta = x/1=x`

if we perceive the Cartesian plane and the angle formed between the x-axis and the line in question (the radius), we can work from the Pythagorean Theorem using `x^2 +y^2 = r^2` .

In a triangle `a^2+b^2 =c^2` so similarly, we have a connection. To simplify the explanation, we will use the 3:4:5 triangle remembering that each one is a ratio, as with the unit circle.

`therefore (3/5)^2 + (4/5)^2 = c^2` where c is the hypotenuse. Simplified:

` 9/25 +16/25 = 1`

To equate this with the first identity:

`sin^2theta+cos^2theta=1`  the connection is now apparent and the reasoning behind the "Pythagorean Identities" is explained.

Ans: Pythagorean Identities are based on the Pythagorean Theorem and the concept of the unit circle and the hypotenuse of one.

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