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Question

Find the polar equation of the tine tangent to the polar curve r=cosθ+sinθr=\cos \theta+\sin \theta at the origin, and then find the slope of this tangent line.

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To find the equation of the line tangent to the polar curve r=cosθ+sinθr=\cos \theta+\sin \theta at the origin we need to solve the equation r=0r=0 for θ\theta:

r=0cosθ=sinθcos(θ)=sin(θ)θ=π4θ=π4θ=3π4\begin{align*} r&=0\Leftrightarrow\\ \cos \theta&=-\sin \theta\Leftrightarrow\\ \cos (-\theta)&=\sin (-\theta)\Leftrightarrow\\ -\theta&=\frac{\pi}{4}\Leftrightarrow \pmb{\theta=-\frac{\pi}{4}\,\,\lor \,\, \theta=\frac{3\pi}{4}} \end{align*}

Now that we know that the equation of the tangent line at the origin is π4-\frac{\pi}{4} we see that the slope of the line is equal to

tan(π4)=1\boxed{\tan \left( -\frac{\pi}{4}\right)=-1}

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