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Prove the identity. \[ \frac{\cos ^{2} \alpha+\cot \alpha}{\cos ^{2} \alpha-\cot \alpha}=\frac{\cos ^{2} \alpha \tan \alpha+1}{\cos ^{2} \alpha \tan \alpha-1} \] To verify the identity, work each side separately until you obtain the same expression. Start with the left side. Choose the correct step and transform the expression according to the step chosen. \( \frac{\cos ^{2} \alpha+\cot \alpha}{\cos ^{2} \alpha-\cot \alpha}=\square \)

Ask by Norris Gordon. in the United States
Feb 26,2025

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To prove the identity, replace cotα with cosα/sinα and combine the terms by multiplying by sinα, resulting in (cos²α·tanα + 1) / (cos²α·tanα − 1).

Solución

We wish to show that   (cos²α + cotα) / (cos²α − cotα) = (cos²α·tanα + 1) / (cos²α·tanα − 1). One good first step is to rewrite cotα in terms of sine and cosine. Recall that   cotα = cosα/sinα. Substitute this into the original expression:   (cos²α + cosα/sinα) / (cos²α − cosα/sinα). Now, both the numerator and the denominator contain a sum (or difference) of two terms with different denominators. To combine them, multiply the numerator and the denominator by sinα (which is allowed as long as sinα ≠ 0):   [(cos²α·sinα + cosα) / sinα] ÷ [(cos²α·sinα − cosα) / sinα]. Since both numerator and denominator are divided by sinα, it cancels:   (cos²α·sinα + cosα) / (cos²α·sinα − cosα). Next, factor cosα from the numerator and denominator:   = cosα (cosα·sinα + 1) / [cosα (cosα·sinα − 1)]. Cancel the common factor cosα (provided cosα ≠ 0):   = (cosα·sinα + 1) / (cosα·sinα − 1). At this point, note that   cosα·sinα = cos²α·(sinα/cosα) = cos²α·tanα. Thus, we can rewrite the expression as:   = (cos²α·tanα + 1) / (cos²α·tanα − 1). This is exactly the right side of the identity. In summary, the key step was:   Replace cotα with cosα/sinα and combine the terms by multiplying by sinα. This completes the proof of the given trigonometric identity.

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The Deep Dive

To verify the identity, we start with the left side: \[ \frac{\cos^{2} \alpha + \cot \alpha}{\cos^{2} \alpha - \cot \alpha} \] We know that: \[ \cot \alpha = \frac{\cos \alpha}{\sin \alpha} \] Substituting this into the expression gives us: \[ = \frac{\cos^{2} \alpha + \frac{\cos \alpha}{\sin \alpha}}{\cos^{2} \alpha - \frac{\cos \alpha}{\sin \alpha}} \] Now, to combine the fractions, we will multiply both the numerator and denominator by \(\sin \alpha\): \[ = \frac{\sin \alpha (\cos^{2} \alpha) + \cos \alpha}{\sin \alpha (\cos^{2} \alpha) - \cos \alpha} \] This transforms our expression to: \[ = \frac{\sin \alpha \cos^{2} \alpha + \cos \alpha}{\sin \alpha \cos^{2} \alpha - \cos \alpha} \] Next, factor out a \(\cos \alpha\) in the numerator and the denominator: \[ = \frac{\cos \alpha \left( \sin \alpha \cos \alpha + 1 \right)}{\cos \alpha \left( \sin \alpha \cos \alpha - 1 \right)} \] Cancel the \(\cos \alpha\) (assuming \(\cos \alpha \neq 0\)): \[ = \frac{\sin \alpha \cos \alpha + 1}{\sin \alpha \cos \alpha - 1} \] Now we have simplified the left side to: \[ \frac{\sin \alpha \cos \alpha + 1}{\sin \alpha \cos \alpha - 1} \] Next, we should simplify the right side starting from: \[ \frac{\cos^{2} \alpha \tan \alpha + 1}{\cos^{2} \alpha \tan \alpha - 1} \] Using \(\tan \alpha = \frac{\sin \alpha}{\cos \alpha}\), we substitute: \[ = \frac{\cos^{2} \alpha \cdot \frac{\sin \alpha}{\cos \alpha} + 1}{\cos^{2} \alpha \cdot \frac{\sin \alpha}{\cos \alpha} - 1} \] This simplifies to: \[ = \frac{\sin \alpha \cos \alpha + 1}{\sin \alpha \cos \alpha - 1} \] As we can see, both sides now reduce to: \[ \frac{\sin \alpha \cos \alpha + 1}{\sin \alpha \cos \alpha - 1} \] Thus, we have verified the identity. Therefore, the identity is proven to be true.

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