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Prove the trigonometric identity $1-2\cos\left(x\right)^2=\frac{\tan\left(x\right)^2-1}{\tan\left(x\right)^2+1}$

Step-by-step Solution

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Final Answer

true

Step-by-step Solution

Specify the solving method

1

Starting from the right-hand side (RHS) of the identity

$\frac{\tan\left(x\right)^2-1}{\tan\left(x\right)^2+1}$

Use the trigonometric identities: $\displaystyle\tan\left(\theta\right)=\frac{\sin\left(\theta\right)}{\cos\left(\theta\right)}$ and $\displaystyle\cot\left(\theta\right)=\frac{\cos\left(\theta\right)}{\sin\left(\theta\right)}$

$\frac{\frac{\sin\left(x\right)^2}{\cos\left(x\right)^2}-1}{\frac{\sin\left(x\right)^2}{\cos\left(x\right)^2}+1}$
2

Rewrite $\frac{\tan\left(x\right)^2-1}{\tan\left(x\right)^2+1}$ in terms of sine and cosine functions

$\frac{\frac{\sin\left(x\right)^2}{\cos\left(x\right)^2}-1}{\frac{\sin\left(x\right)^2}{\cos\left(x\right)^2}+1}$

Combine $\frac{\sin\left(x\right)^2}{\cos\left(x\right)^2}+1$ in a single fraction

$\frac{\frac{\sin\left(x\right)^2}{\cos\left(x\right)^2}-1}{\frac{\sin\left(x\right)^2+\cos\left(x\right)^2}{\cos\left(x\right)^2}}$

Applying the pythagorean identity: $\sin^2\left(\theta\right)+\cos^2\left(\theta\right)=1$

$\frac{\frac{\sin\left(x\right)^2}{\cos\left(x\right)^2}-1}{\frac{1}{\cos\left(x\right)^2}}$
Why is sin(x)^2 + cos(x)^2 = 1 ?
3

Simplify the fraction $\frac{\sin\left(x\right)^2}{\cos\left(x\right)^2}+1$ into $\frac{1}{\cos\left(x\right)^2}$

$\frac{\frac{\sin\left(x\right)^2}{\cos\left(x\right)^2}-1}{\frac{1}{\cos\left(x\right)^2}}$
4

Divide fractions $\frac{\frac{\sin\left(x\right)^2}{\cos\left(x\right)^2}-1}{\frac{1}{\cos\left(x\right)^2}}$ with Keep, Change, Flip: $a\div \frac{b}{c}=\frac{a}{1}\div\frac{b}{c}=\frac{a}{1}\times\frac{c}{b}=\frac{a\cdot c}{b}$

$\left(\frac{\sin\left(x\right)^2}{\cos\left(x\right)^2}-1\right)\cos\left(x\right)^2$
5

Combine all terms into a single fraction with $\cos\left(x\right)^2$ as common denominator

$\frac{\sin\left(x\right)^2-\cos\left(x\right)^2}{\cos\left(x\right)^2}\cos\left(x\right)^2$

Multiplying the fraction by $\cos\left(x\right)^2$

$\frac{\left(\sin\left(x\right)^2-\cos\left(x\right)^2\right)\cos\left(x\right)^2}{\cos\left(x\right)^2}$

Simplify the fraction $\frac{\left(\sin\left(x\right)^2-\cos\left(x\right)^2\right)\cos\left(x\right)^2}{\cos\left(x\right)^2}$ by $\cos\left(x\right)^2$

$\sin\left(x\right)^2-\cos\left(x\right)^2$
6

Multiplying the fraction by $\cos\left(x\right)^2$

$\sin\left(x\right)^2-\cos\left(x\right)^2$
7

Applying the trigonometric identity: $\sin\left(\theta \right)^2 = 1-\cos\left(\theta \right)^2$

$1-\cos\left(x\right)^2-\cos\left(x\right)^2$
Why is 1 - cos(x)^2 = sin(x)^2 ?
8

Combining like terms $-\cos\left(x\right)^2$ and $-\cos\left(x\right)^2$

$1-2\cos\left(x\right)^2$
9

Since we have reached the expression of our goal, we have proven the identity

true

Final Answer

true

Explore different ways to solve this problem

Solving a math problem using different methods is important because it enhances understanding, encourages critical thinking, allows for multiple solutions, and develops problem-solving strategies. Read more

Prove from LHS (left-hand side)Express everything into Sine and Cosine

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Function Plot

Plotting: $true$

Main Topic: Trigonometric Identities

In mathematics, trigonometric identities are equalities that involve trigonometric functions and are true for every single value of the occurring variables where both sides of the equality are defined.

Used Formulas

3. See formulas

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