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Prove that -

\sin {}^(2) (\theta) + \cos {}^(2) (\theta) = 1

ty !​

User Adam D
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1 Answer

13 votes

Answer:

Explanation

There are many ways to prove this identity. But, I will use the simplest one. If you have a right triangle, the length of the horizontal side is x, the length of the vertical side is y, and the hypotenuse is r. So, the relation between x and r can be written in cosine form:


\cos\theta=(x)/(r)

The relation between y and r can be written in sine form:


\sin\theta=(y)/(r)

Pythagoras told you that for the right triangle
x^(2)+y^(2)=r^(2) should be satisfied. Substitute x and y from the cosine and sine form:


(r\cos\theta)^(2)+(r\sin\theta)^(2)=r^(2)


r^(2)\cos^(2)\theta+r^(2)\sin^(2)\theta =r^(2)

the square of r will cancel out, and you can get your identity:


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

User Fan Cheung
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