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Calculus Level 5

Find the slope of the tangent to the curve y = 1 2 x + 3 y=\dfrac{1}{2x}+3 at the point where x = 1 x=-1 . Find the angle which this tangent makes with the curve y = 2 x 2 + 2 y=2x^2+2 , where x < 0 x < 0 . Submit your answer as the angle in degrees. You may use the fact that arctan ( 1 2 ) 2 6 \arctan \left( \frac{1}{2} \right)\approx 26^\circ .


The answer is 19.

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1 solution

The slope of the tangent to y = 1 2 x + 3 y = \dfrac 1{2x}+3 at x = 1 x=-1 is d y d x x = 1 = 1 2 x 2 x = 1 = 1 2 \dfrac {dy}{dx} \bigg|_{x=-1} = - \dfrac 1{2x^2} \bigg|_{x=-1} = - \dfrac 12 .

Then y ( 1 ) = 1 2 ( 1 ) + 3 = 5 2 y(-1) = \dfrac 1{2(-1)}+3 = \dfrac 52 and the equation of the tangent is y 5 2 x + 1 = 1 2 \dfrac {y-\frac 52}{x +1} = - \dfrac 12 , y = 2 x 2 \implies y = 2 - \dfrac x2 .

When the tangent meet the other curve y = 2 x 2 + 2 y = 2x^2 + 2 :

2 x 2 = 2 x 2 + 2 2 x 2 + x 2 = 0 4 x 2 + x = 0 x ( 4 x + 1 ) = 0 x = 1 4 Since x < 0 \begin{aligned} 2 - \frac x2 & = 2x^2+2 \\ 2x^2 + \frac x2 & = 0 \\ 4x^2 + x & = 0 \\ x(4x+1) = 0 \\ \implies x & = - \frac 14 & \small \color{#3D99F6} \text{Since} x < 0 \end{aligned}

The slope of the curve y = 2 x 2 + 2 y = 2x^2 + 2 at x = 1 4 x = -\dfrac 14 is d y d x x = 1 4 = 4 x x = 1 4 = 1 \dfrac {dy}{dx} \bigg|_{x=-\frac 14} = 4x \bigg|_{x=-\frac 14} = - 1 .

The angle between the tangent and the curve is tan 1 1 tan 1 1 2 4 5 2 6 = 19 \tan^{-1} 1 - \tan^{-1} \dfrac 12 \approx 45^\circ - 26^\circ = \boxed{19}^\circ

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