# Chapter 5 - Section 5.2 - Sum and Difference Formulas - Exercise Set - Page 670: 69

The result of $\cos \left( \alpha +\beta \right)\cos \beta +\sin \left( \alpha +\beta \right)\sin \beta$ is $\cos \alpha$.

#### Work Step by Step

Let us consider the given expression, $\cos \left( \alpha +\beta \right)\cos \beta +\sin \left( \alpha +\beta \right)\sin \beta$ By using the trigonometric identities, $\cos \left( \alpha +\beta \right)=\cos \alpha \cos \beta -\sin \alpha \sin \beta$ $\sin \left( \alpha +\beta \right)=\sin \alpha \cos \beta +\cos \alpha \sin \beta$ Now, the above expression can be further simplified as: \begin{align} & \cos \left( \alpha +\beta \right)\cos \beta +\sin \left( \alpha +\beta \right)\sin \beta =\left\{ \left( \cos \alpha \cos \beta -\sin \alpha \sin \beta \right)\cos \beta +\left( \sin \alpha \cos \beta +\cos \alpha \sin \beta \right)\sin \beta \right\} \\ & =\left\{ \cos \alpha {{\cos }^{2}}\beta -\sin \alpha \sin \beta \cos \beta +\sin \alpha \cos \beta \sin \beta +\cos \alpha {{\sin }^{2}}\beta \right\} \\ & =\cos \alpha {{\cos }^{2}}\beta +\cos \alpha {{\sin }^{2}}\beta \\ & =\cos \alpha \left( {{\cos }^{2}}\beta +{{\sin }^{2}}\beta \right) \end{align} by using the identity ${{\sin }^{2}}x+{{\cos }^{2}}x=1$ \begin{align} & \cos \alpha \left( {{\cos }^{2}}\beta +{{\sin }^{2}}\beta \right)=\cos \alpha \left( {{\cos }^{2}}\beta +{{\sin }^{2}}\beta \right) \\ & =\cos \alpha \left( 1 \right) \\ & =\cos \alpha \end{align} Thus, $\cos \left( \alpha +\beta \right)\cos \beta +\sin \left( \alpha +\beta \right)\sin \beta$ can be simplified as $\cos \alpha$. Hence, the result of $\cos \left( \alpha +\beta \right)\cos \beta +\sin \left( \alpha +\beta \right)\sin \beta$ is $\cos \alpha$.

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