Illustrate the parabolas $y=x^2$ and $y = x^2 - 2x + 2$ and state if there is a line that is tangent to both curves.
If so, find its equation. If not, why not?
For $\displaystyle y = x^2$,
$\displaystyle\qquad \frac{dy}{dx} = \frac{d}{dx}(x^2) = 2x \quad \Longleftarrow \text{Equation 1}$
For $\displaystyle y = x^2-2x +2$,
$\displaystyle\qquad \frac{dy}{dx} = \frac{d}{dx}(x^2) - 2 \frac{dy}{dx}(x) = \frac{d}{dx}(2)
= 2x - 2 \quad \Longleftarrow \text{Equation 2}$
Let $y = ax + b$ be the equation of the line that is tangent to $ y = x^2$ at $(x_1,x_1^2)$
and $y = x^3 - 2x + 2$ at $(x_2, x_2^2 - 2x_2 + 2)$.
By using the equation of the line and Equations 1 and 2 we get
$
\begin{equation}
\begin{aligned}
ax+1 + b &= x_1^2 && \Longleftarrow \text{Equation 3}\\
\\
ax_2 + b &= x_2^2 - 2x_2 + 2 && \Longleftarrow \text{Equation 4}
\end{aligned}
\end{equation}
$
Also, recall that the slope $a$ is equal to the derivative of the curve. So,
$
\begin{equation}
\begin{aligned}
a & = 2x_1 &;&& x_1 &= \frac{a}{2}\\
\\
a &= 2x_2-2 &;&& x_2 &= \frac{a}{2} +1
\end{aligned}
\end{equation}
$
Using these equations together with Equations 3 and 4 we get
From Equation 3:
$
\begin{equation}
\begin{aligned}
a \left(\frac{a}{2}\right) + b & = \left( \frac{a}{2} \right)^2\\
\\
\frac{a^2}{2}+b &= \frac{a^2}{4}\\
\\
b & = \frac{a^2}{4} - \frac{a^2}{2}\\
\\
b &= - \frac{a^2}{4}
\end{aligned}
\end{equation}
$
From Equation 4:
$
\begin{equation}
\begin{aligned}
a\left(\frac{a}{2}+1\right) + b & = \left( \frac{a}{2} + 1 \right)^2 - 2 \left( \frac{a}{2} +1 \right) +1\\
\\
\frac{a^2}{2} + a + b &= \frac{a^2}{4} + \cancel{a} + 1 - \cancel{a} - \cancel{2} + \cancel{2}\\
\\
b &= \frac{a^2}{4} - \frac{a^2}{2} - a + 1\\
\\
b &= \frac{-a^2}{4} - a +1
\end{aligned}
\end{equation}
$
Solving for $a$
$
\begin{equation}
\begin{aligned}
\cancel{\frac{-a^2}{4}} &= \cancel{\frac{-a^2}{4}} - a + 1\\
\\
a & = 1
\end{aligned}
\end{equation}
$
Solving for $b$
$
\begin{equation}
\begin{aligned}
b &= - \frac{a^2}{4} = -\frac{(1)^2}{4}\\
b &= - \frac{1}{4}
\end{aligned}
\end{equation}
$
Plugging the values of $a$ and $b$ to the equation of the line we get
$
\begin{equation}
\begin{aligned}
y &= ax + b\\
y &= (1)x + \left( - \frac{1}{4} \right)\\
y &= x - \frac{1}{4}
\end{aligned}
\end{equation}
$
Therefore, the equation of the line that is tangent to both curve is $\displaystyle y = x - \frac{1}{4}$
Tuesday, October 18, 2016
Single Variable Calculus, Chapter 3, 3.3, Section 3.3, Problem 102
Subscribe to:
Post Comments (Atom)
Why is the fact that the Americans are helping the Russians important?
In the late author Tom Clancy’s first novel, The Hunt for Red October, the assistance rendered to the Russians by the United States is impor...
-
There are a plethora of rules that Jonas and the other citizens must follow. Again, page numbers will vary given the edition of the book tha...
-
The only example of simile in "The Lottery"—and a particularly weak one at that—is when Mrs. Hutchinson taps Mrs. Delacroix on the...
-
A good thesis statement presents a claim (an interpretive stance on a story that can be defended using textual evidence) and is a position w...
-
The given two points of the exponential function are (2,24) and (3,144). To determine the exponential function y=ab^x plug-in the given x an...
-
What does the hot air balloon symbolize? To the Assad son who buys the hot air balloon, it symbolizes a kind of whimsy that he can afford. B...
-
The play Duchess of Malfi is named after the character and real life historical tragic figure of Duchess of Malfi who was the regent of the ...
-
Allie’s baseball mitt is extremely important to Holden in The Catcher in the Rye. It is a symbol of Allie since it was important to his brot...
No comments:
Post a Comment