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Concepts and Vocabulary
True or False \({\mathop{\rm{csch}}}x=\dfrac{1}{\cosh x}\).
In terms of \(\sinh x\) and \(\cosh x,\) \(\tanh x=\) _________________.
True or False The domain of \(y=\cosh x\) is \([1,\infty)\).
Multiple Choice The function \(y=\cosh x\) is [(a) even, (b) odd, (c) neither].
True or False \(\cosh ^{2}x+\sinh ^{2}x=1\).
True or False \(\dfrac{d}{dx}\sinh x=\dfrac{1}{\ \sqrt{x^{2}+1}}\).
True or False The function \(y=\sinh ^{-1}x\) is defined for all real numbers \(x\).
True or False \(\dfrac{d}{dx}\sinh^{-1}x=\cosh ^{-1}x\)
Skill Building
In Problems 9–14, find the exact value of each expression.
\({\mathop{\rm{csch}}}(\ln 3) \)
\({\mathop{\rm{sech}}}( \ln 2) \)
\(\cosh ^{2}(5) -\sinh ^{2}(5) \)
\(\cosh ( -\!\ln 2) \)
\(\tanh 0\)
\(\sinh \left(\ln \dfrac{1}{2}\right) \)
In Problems 15–24, establish each identity.
\(\tanh ^{2}x+{\mathop{\rm{sech}}}^{2}x=1\)
\(\coth ^{2}x-{\mathop{\rm{csch}}}^{2}x=1\)
\(\sinh (-A)=-\sinh A\)
\(\cosh (-A)=\cosh A\)
\(\sinh (A+B)=\sinh A\cosh B+\cosh A\sinh B\)
\(\cosh (A+B)=\cosh A\cosh B+\sinh A\sinh B\)
\(\sinh (2x) =2 \sinh x \cosh x\)
\(\cosh (2x) =\cosh ^{2}x+\sinh ^{2}x\)
\(\cosh (3x) =4\cosh ^{3}x-3\cosh x\)
\(\tanh (2x) = \dfrac{2\tanh x}{1+\tanh ^{2}x}\)
In Problems 25–48, find \(y^\prime \).
\(y=\sinh (3x) \)
\(y=\cosh \dfrac{x}{2}\)
\(y=\cosh (x^{2}+1)\)
\(y=\cosh (2x^{3}-1)\)
\(y=\coth \dfrac{1}{x}\)
\(y=\tanh ( x^{2})\)
\(y=\sinh x \cosh (4x) \)
\(y=\sinh (2x) \cosh (-x)\)
\(y=\cosh ^{2}x\)
\(y=\tanh ^{2}x\)
\(y=e^{x}\cosh x\)
\(y=e^{x}(\cosh x+\sinh x)\)
\(y=x^{2}\;{\mathop{\rm{sech}}}\;x\)
\(y=x^{3}\tanh x\)
\(y=\cosh ^{-1}( 4x)\)
\(y=\sinh ^{-1}(3x)\)
\(y=\tanh ^{-1}(x^{2}-1)\)
\(y=\cosh ^{-1}(2x+1)\)
\(y=x\sinh ^{-1}x\)
\(y=x^{2}\cosh ^{-1}x\)
\(y=\tanh ^{-1}(\tan x)\)
\(y=\sinh ^{-1}(\sin x)\)
\(y=\cosh ^{-1}\big( \ \sqrt{x^{2}-1}\big)\), \(x>\ \sqrt{2}\)
\(y=\sinh ^{-1}\big( \ \sqrt{x^{2}+1}\big) \)
Applications and Extensions
Taylor Polynomial Write the Taylor Polynomial \(P_{6}( x) \) for \(g(x)=\cosh \,x\) at \(0.\)
Taylor Polynomial Write the Taylor Polynomial \(P_{7}( x) \) for \(f(x)=\tanh \,x\) at \(0.\)
Catenary A cable is suspended between two supports of the same height that are \(100{\,{\rm{m}}}\) apart. If the supports are placed at \(( -50,0) \) and \(( 50,0)\), the equation that models the height of the cable is \(y=12\cosh \dfrac{x}{12}+20\). Find the angle \(\theta \) at which the cable meets each support.
Catenary A town hangs strings of holiday lights across the road between utility poles. Each set of poles is \(12{\,{\rm{m}}}\) apart. The strings hang in catenaries modeled by \(y=15\cosh \dfrac{x}{15}-10\).
Catenary The famous Gateway Arch to the West in St. Louis, Missouri, is constructed in the shape of a modified inverted catenary. (Modified because the weight is not evenly dispersed throughout the arch.) If \(y\) is the height of the arch (in feet) and \(x=0\) corresponds to the center of the arch (its highest point), an equation for the arch is given by \[ y=-68.767\cosh \left( \dfrac{0.711x}{68.767}\right) + 693.859 {\,{\rm{ft}}}, \]
Establish the identity \((\cosh x+\sinh x)^{n}=\cosh (nx) +\sinh ( nx)\) for any real number \(n\).
Show that \(\dfrac{d}{dx}\tanh x={\mathop{\rm{sech}}}^{2}x\).
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Show that \(\dfrac{d}{dx}\coth x=-{\mathop{\rm{csch}}}^{2}x\).
Show that \(\dfrac{d}{dx}{\mathop{\rm{sech}}}x=-{\mathop{\rm{sech}}}x \tanh x\)
Show that \(\dfrac{d}{dx}{\mathop{\rm{csch}}}x=-{\mathop{\rm{csch}}} x \coth x\)
Show that \(\tanh ^{-1}x=\dfrac{1}{2}\ln \left(\dfrac{1+x}{1-x}\right)\), \(-1<x<1\).
Show that \(\cosh ^{-1}x=\ln (x+\ \sqrt{x^{2}-1})\), \(x ≥ 1\).
Show that \(\coth ^{-1}x=\dfrac{1}{2}\ln \left( \dfrac{x+1}{x-1}\right)\), \(\vert x\vert >1\).
Challenge Problems
In Problems 64 and 65, find each limit.
\(\lim\limits_{x\rightarrow 0}\left( \dfrac{\sinh x}{x}\right) \)
\(\lim\limits_{x\rightarrow 0}\left( \dfrac{\cosh x-1}{x}\right) \)
What happens if you try to find the derivative of \(f( x) =\sin ^{-1}( \cosh x) \)? Explain why this occurs.
Let \(f(x)=x\sinh ^{-1}x\).