The blanks are filled by 
Laws of Cosine, 
Laws of Sine,
Pythagorean Theorem,
Substitution method,
Laws of Exponents
respectively.
What are the Laws of Cosine? 
A triangle with sides x, y, and hypotenuse r. 
Angle θ is opposite to side y.
[tex]cos(\theta)=\frac{\mbox{adjacent side}}{\mbox{hypotenuse}}=\frac{x}{r}[/tex]  
What are the Laws of Sine? 
A triangle with sides x, y, and hypotenuse r. 
Angle θ is opposite to side y.
[tex]sin(\theta)=\frac{\mbox{opposite side}}{\mbox{hypotenuse}}[/tex]
What is Pythagorean Identity? 
A triangle with sides x, y, and hypotenuse r. 
Angle θ is opposite to side y.
Then the property 
x²+y²=r²
is called the Pythagorean Identity. 
How to fill the blanks? 
Given the right triangle shown below, prove cos²(θ) + sin²(θ) = 1.
A triangle with sides x, y, and hypotenuse r. 
Angle θ is opposite to side y.
By Laws of Cosine
, cos(θ) = x/r, and
by Laws of Sine
, sin(θ) = y/r.
Multiplying both sides of the above equations by r, we get that
 x = r cos(θ) and 
y = r sin(θ).
The Pythagorean Theorem states that x² + y² = r².
By Substitution method , we have [r cos(θ)]² + [r sin(θ)]²= r².
Applying the Laws of Exponents, the equation can be written as r² cos²(θ) + r² sin²(θ) = r².
Dividing both sides of the equation by r² results in cos²(θ) + sin²(θ) = 1.
Hence, the blanks are filled by 
Laws of Cosine, 
Laws of Sine,
Pythagorean Theorem,
Substitution method,
Laws of Exponents
respectively.
To learn more about Pythagorean Identity visit- https://brainly.com/question/15190643
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