The given curve has equation
<em>r(θ)</em> = 9 + 8 cos(<em>θ</em>)
and its derivative is
d<em>r</em>/d<em>θ</em> = -8 sin(<em>θ</em>)
When <em>θ</em> = <em>π</em>/3, we have <em>r</em> (<em>π</em>/3) = 13, and d<em>r</em>/d<em>θ</em> (<em>π</em>/3) = -4√3.
Differentiate these with respect to <em>θ</em> :
d<em>y</em>/d<em>θ</em> = d<em>r</em>/d<em>θ</em> sin(<em>θ</em>) + <em>r(θ)</em> cos(<em>θ</em>)
d<em>x</em>/d<em>θ</em> = d<em>r</em>/d<em>θ</em> cos(<em>θ</em>) - <em>r(θ</em>) sin(<em>θ</em>)
In polar coordinates, we have
<em>y(θ)</em> = <em>r(θ)</em> sin(<em>θ</em>)
<em>x(θ)</em> = <em>r(θ)</em> cos(<em>θ</em>)
and when <em>θ</em> = <em>π</em>/3, we have <em>y</em> (<em>π</em>/3) = 13√3/2 and <em>x</em> (<em>π</em>/3) = 13/2.
The slope of the tangent line to the curve is d<em>y</em>/d<em>x</em>. By the chain rule,
d<em>y</em>/d<em>x</em> = d<em>y</em>/d<em>θ</em> • d<em>θ</em>/d<em>x</em> = (d<em>y</em>/d<em>θ</em>) / (d<em>x</em>/d<em>θ</em>)
d<em>y</em>/d<em>x</em> = (d<em>r</em>/d<em>θ</em> sin(<em>θ</em>) + <em>r(θ)</em> cos(<em>θ</em>)) / (d<em>r</em>/d<em>θ</em> cos(<em>θ</em>) - <em>r(θ</em>) sin(<em>θ</em>))
When <em>θ</em> = <em>π</em>/3, the slope is
d<em>y</em>/d<em>x</em> = (-4√3 sin(<em>π</em>/3) + 13 cos(<em>π</em>/3)) / (-4√3 cos(<em>π</em>/3) - 13 sin(<em>π</em>/3))
d<em>y</em>/d<em>x</em> = (-4√3 (√3/2) + 13 (1/2)) / (-4√3 (1/2) - 13 (√3/2))
d<em>y</em>/d<em>x</em> = - 1/(17√3)
So, the tangent line has slope -1/(17√3) and passes through (13/2, 13√3/2). Using the point-slope formula, its equation is
<em>y</em> - 13√3/2 = -1/(17√3) (<em>x</em> - 13/2)
<em>y</em> = -(<em>x</em> - 338)/(17√3)
![\bf \begin{array}{lllll} &x_1&y_1&x_2&y_2\\ % (a,b) &({{ -3}}\quad ,&{{ 2}})\quad % (c,d) &({{ -4}}\quad ,&{{ 0}}) \end{array} \\\\\\ % slope = m slope = {{ m}}= \cfrac{rise}{run} \implies \cfrac{{{ y_2}}-{{ y_1}}}{{{ x_2}}-{{ x_1}}}\implies \cfrac{0-2}{-4-(-3)}\implies \cfrac{0-2}{-4+3} \\\\\\ \cfrac{-2}{-1}\implies 2 \\\\\\ % point-slope intercept \stackrel{\textit{point-slope form}}{y-{{ y_1}}={{ m}}(x-{{ x_1}})}\implies y-2=2[x-(-3)] \\\\\\ y-2=2(x+3)\implies y-2=2x+6\implies y=2x+8](https://tex.z-dn.net/?f=%5Cbf%20%5Cbegin%7Barray%7D%7Blllll%7D%0A%26x_1%26y_1%26x_2%26y_2%5C%5C%0A%25%20%20%20%28a%2Cb%29%0A%26%28%7B%7B%20-3%7D%7D%5Cquad%20%2C%26%7B%7B%202%7D%7D%29%5Cquad%20%0A%25%20%20%20%28c%2Cd%29%0A%26%28%7B%7B%20-4%7D%7D%5Cquad%20%2C%26%7B%7B%200%7D%7D%29%0A%5Cend%7Barray%7D%0A%5C%5C%5C%5C%5C%5C%0A%25%20slope%20%20%3D%20m%0Aslope%20%3D%20%7B%7B%20m%7D%7D%3D%20%5Ccfrac%7Brise%7D%7Brun%7D%20%5Cimplies%20%0A%5Ccfrac%7B%7B%7B%20y_2%7D%7D-%7B%7B%20y_1%7D%7D%7D%7B%7B%7B%20x_2%7D%7D-%7B%7B%20x_1%7D%7D%7D%5Cimplies%20%5Ccfrac%7B0-2%7D%7B-4-%28-3%29%7D%5Cimplies%20%5Ccfrac%7B0-2%7D%7B-4%2B3%7D%0A%5C%5C%5C%5C%5C%5C%0A%5Ccfrac%7B-2%7D%7B-1%7D%5Cimplies%202%0A%5C%5C%5C%5C%5C%5C%0A%25%20point-slope%20intercept%0A%5Cstackrel%7B%5Ctextit%7Bpoint-slope%20form%7D%7D%7By-%7B%7B%20y_1%7D%7D%3D%7B%7B%20m%7D%7D%28x-%7B%7B%20x_1%7D%7D%29%7D%5Cimplies%20y-2%3D2%5Bx-%28-3%29%5D%0A%5C%5C%5C%5C%5C%5C%0Ay-2%3D2%28x%2B3%29%5Cimplies%20y-2%3D2x%2B6%5Cimplies%20y%3D2x%2B8)