Question #173512
  1. Is sec (−𝜽) equal to sec 𝜽 or - sec 𝜽? How do you know? 
  2. Verify the identity 1 - sin2 x cot2 x = sin2 x. Is there more than one way to verify the identity? If so, tell which way you think is easier and why. 
  3. Describe what is wrong with the simplification shown.

cos x - cos x sin² x = cos x - cos x (1 + cos² x)

          = cos x - cos x - cos³ x

          = -cos³ x

4. John said 𝒔𝒊𝒏 𝒙 + 𝒄𝒐𝒔 𝒙 = 𝟐 has no solution. Do you agree with John? Explain why or why not?


Verify identity:

  1. 𝒄𝒐𝒕²𝜽+𝟏 / 𝒄𝒐𝒕²𝜽 ≡ 𝒔𝒆𝒄²𝜽 
  2.  (𝒄𝒔𝒄²𝜽 − 𝟏)𝒔𝒊𝒏²𝜽≡ 𝒄𝒐𝒔²𝜽
  3. 𝟏 − 𝒔𝒆𝒄 𝜶 𝒄𝒐𝒔 𝜶 ≡ 𝒕𝒂𝒏 𝜶 𝒄𝒐𝒕 𝜶 − 1
  4. 𝒕𝒂𝒏 𝑨+𝒄𝒐𝒕 𝑨 / 𝒔𝒆𝒄 𝑨 𝒄𝒔𝒄 𝑨 ≡ 1
  5. 𝟏 + 𝟐 𝒕𝒂𝒏²𝜽 ≡ 𝒔𝒆𝒄𝜽 − 𝒕𝒂𝒏𝜽
1
Expert's answer
2021-03-31T13:53:17-0400

Solution.

1.

sec(θ)=secθ,\sec{(-\theta)}=\sec{\theta},

because secθ=1cosθ\sec\theta=\frac{1}{\cos \theta} and cos(θ)=cosθ.\cos(-\theta)=\cos \theta.

2.

1sin2xcot2x=1sin2xcos2xsin2x==1cos2x=sin2x.1-\sin^2x\cot^2x=1-\sin^2x\frac{\cos^2x}{\sin^2x}=\newline =1-\cos^2x=\sin^2x.

As for me, this way is easier, because it is a way of proving by definition.

3.

   cosxcosxsin2x=cosxcosx(1cos2x)==cosxcosx+cos3x=cos3x.\cos x-\cos x\sin^2x=\cos x-\cos x(1-\cos^2x)=\newline =\cos x-\cos x+\cos^3x=\cos^3x.

4.

I agree with John, because this equation has no solution.

sinx+cosx=2,\sin x+\cos x=2,

2sin(x+π4)=2,\sqrt{2}\sin(x+\frac{\pi}{4})=2,

sin(x+π4)=22,\sin(x+\frac{\pi}{4})=\frac{2}{\sqrt{2}},

and 22>1.\frac{2}{\sqrt{2}}>1.

This equation has no real solutions.

Verify identity:

1.

cot2θ+1cot2θ=cot2θ+tan2θ,sec2θ=1cos2θ=1+tan2θ.\cot^2\theta+\frac{1}{\cot^2\theta}=\cot^2\theta+\tan^2\theta,\newline \sec^2\theta=\frac{1}{\cos^2\theta}=1+\tan^2\theta.

But 1cot2θ.1\neq \cot^2\theta. This equality is not an identity.

cot2θ+1cot2θsec2θ.\cot^2\theta+\frac{1}{\cot^2\theta}\neq\sec^2\theta.

2.

(csc2θ1)sin2θ=(1sin2θ1)sin2θ==1sin2θsin2θsin2θ=1sin2θ=cos2θ.(\csc^2\theta-1)\sin^2\theta=(\frac{1}{\sin^2\theta}-1)\sin^2\theta=\newline=\frac{1-\sin^2\theta}{\sin^2\theta}\sin^2\theta=1-\sin^2\theta=\cos^2\theta.

3.

1secαcosα=0,1-\sec\alpha\cos\alpha=0, and tanαcotα1=0.\tan\alpha\cot\alpha-1=0.

So, 1secαcosα=tanαcotα1.1-\sec\alpha\cos\alpha=\tan\alpha\cot\alpha-1.

4.

tanA+cotAsecAcscA=tanA+cosAcosAsinAsinA==tanA+cos2A=sinA+cos3AcosA1.\tan A+\frac{\cot A}{\sec A\csc A}=\tan A+\frac{\cos A\cdot \cos A\sin A}{\sin A}= \newline=\tan A+\cos^2A=\frac{\sin A+\cos^3A}{\cos A}\neq 1.

This equality is not an identity.

5.

sec4θtan4θ=(sec2θtan2θ)(sec2θ+tan2θ)==1sin2θcos2θ1+sin2θcos2θ=cos2θ+sin2θ+sin2θcos2θ=1+2tan2θ.\sec^4\theta-\tan^4\theta=(\sec^2\theta-\tan^2\theta)(\sec^2\theta+\tan^2\theta)=\newline =\frac{1-sin^2\theta}{\cos^2\theta}\cdot\frac{1+sin^2\theta}{\cos^2\theta}= \frac{\cos^2\theta+\sin^2\theta+\sin^2\theta}{\cos^2\theta}=1+2\tan^2\theta.


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