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Fill in the gap.

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if we write $\\boxed{\\var{a}=r\\sin\\alpha}$  and  $\\boxed{\\var{b}=r\\cos\\alpha}$ for some $r$ and $\\alpha$ then we have $(\\var{a})\\cos\\theta+(\\var{b})\\sin\\theta=(r\\sin\\alpha)\\cos\\theta+(r\\cos\\alpha)\\sin\\theta=r\\sin(\\alpha+\\theta)$ . Then the maximum and minimum value will be $r$  and $-r$ since $-1\\leq\\sin(\\alpha+\\theta)\\leq 1$

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To find $r$ we do the following:  $r^2\\sin^2\\alpha=(\\var{a})^2$ and $r^2\\cos^2\\alpha=(\\var{b})^2 \\implies r^2\\sin^2\\alpha+r^2\\cos^2\\alpha=(\\var{a})^2+(\\var{b})^2\\implies r^2(\\sin^2\\alpha+\\cos^2\\alpha)=(\\var{a})^2+(\\var{b})^2\\implies r^2=(\\var{a})^2+(\\var{b})^2\\\\ \\implies r=\\sqrt{({\\var{a})^2+(\\var{b})^2}}=\\var{c}$ 

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The maximum of $(\\var{a})\\cos\\theta +(\\var{b})\\sin\\theta$=[[0]] 

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The minimum of $(\\var{a})\\cos\\theta +(\\var{b})\\sin\\theta$=[[0]]

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