Science:Math Exam Resources/Courses/MATH110/December 2015/Question 09

MATH110 December 2015

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[hide] Question 09
Consider the curve $y=x^{3}-3x+2$. Find the points of the curve where the tangent line is parallel to the secant line that goes through the points $(-2,y(-2))$ and $(2,y(2))$.

Make sure you understand the problem fully: What is the question asking you to do? Are there specific conditions or constraints that you should take note of? How will you know if your answer is correct from your work only? Can you rephrase the question in your own words in a way that makes sense to you?

If you are stuck, check the hints below. Read the first one and consider it for a while. Does it give you a new idea on how to approach the problem? If so, try it! If after a while you are still stuck, go for the next hint.

[show] Hint 1
Recall that the slope secant line that goes through the points $(x_{1},y_{1}),(x_{2},y_{2})$ is given by $m={\frac {y_{1}-y_{2}}{x_{1}-x_{2}}}$, so in the first step, we need to find the y-coordinates of the points using the function equation.

[show] Hint 2
Parallel lines have equal slopes, on the other hand, slope of the tangent line at a point $x$ is equal to $f'(x)$ .

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[show] Solution

Found a typo? Is this solution unclear? Let us know here. Please rate my easiness! It's quick and helps everyone guide their studies. $y=x^{3}-3x+2$ so $y(2)=8-6+2=4,\ y(-2)=-8+6+2=0$. The slope of the secant line through $(2,4)$ and $(-2,0)$ is given by $m={\frac {4-0}{2-(-2)}}=1$ Now we are looking for $x$'s at which the slope of the tangent line is equal to $1$, this means that the derivative of the function at these $x$'s must be equal to $1$, i.e. $f'(x)=1$ which therefore gives that $f'(x)=3x^{2}-3=1\Rightarrow x^{2}={\frac {4}{3}}\Rightarrow x={\frac {2}{\sqrt {3}}},\ x=-{\frac {2}{\sqrt {3}}}$ . The question asks for the points, so we should find the y-coordinate for each $x$ by substituting the $x$ values into the function: $y({\frac {2}{\sqrt {3}}})={\frac {8}{3{\sqrt {3}}}}-3{\frac {2}{\sqrt {3}}}+2={\frac {8}{3{\sqrt {3}}}}-2{\sqrt {3}}+2$, $y(-{\frac {2}{\sqrt {3}}})=-{\frac {8}{3{\sqrt {3}}}}+3{\frac {2}{\sqrt {3}}}+2=-{\frac {8}{3{\sqrt {3}}}}+2{\sqrt {3}}+2$, Answer: $\color {blue}\left(\pm {\frac {2}{\sqrt {3}}},2\pm \left({\frac {8}{3{\sqrt {3}}}}-2{\sqrt {3}}\right)\right)$