Calculus: Early Transcendentals 8th Edition

Published by Cengage Learning
ISBN 10: 1285741552
ISBN 13: 978-1-28574-155-0

Chapter 3 - Section 3.1 - Derivatives of Polynomials and Exponential Functions - 3.1 Exercises - Page 181: 59


There are 2 tangent lines satisfying the requirements: $(l_1):y=3x-3$ and $(l_2):y=3x-7$.

Work Step by Step

$$(s): y=x^3-3x^2+3x-3$$ $$(d): 3x-y=15$$ 1) Find the derivative of $y$ of curve $(s)$: $$y'=\frac{d}{dx}(x^3)-3\frac{d}{dx}(x^2)+3\frac{d}{dx}(x)-\frac{d}{dx}(3)$$ $$y'=3x^2-6x+3$$ 2) The slope of the tangent line $(l)$ of curve $(s)$ at point $A(a,b)$ equals $y'(a)$. Consider line $(d)$: $$(d): 3x-y=15$$ $$(d):y=3x-15$$ So the slope of line $(d)$ equals 3. For 2 lines to be parallel with each other, their slopes must be equal. That means, for the tangent line $(l)$ and line $(d)$ to be parallel with each other, $$y'(a)=3$$ $$3a^2-6a+3=3$$ $$3a^2-6a=0$$ $$3a(a-2)=0$$ $$a=0$$ or $$a=2$$ - For $a=0$, then $b=0^3-3\times0^2+3\times0-3=-3$ - For $a=2$, then $b=2^3-3\times2^2+3\times2-3=-1$ Therefore, there are 2 points $A(0,-3)$ and $B(2,-1)$ at which the tangent line is parallel with line $(d)$. *Point $A(0,-3)$ The equation of the tangent line $(l_1)$ at $A$ would be $$(l_1):(y+3)=3(x-0)$$ $$(l_1):y=3x-3$$ *Point $B(2,-1)$ The equation of the tangent line $(l_2)$ at $B$ would be $$(l_2):(y+1)=3(x-2)$$ $$(l_2):y+1=3x-6$$ $$(l_2):y=3x-7$$
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