009A Sample Final 1, Problem 8

Let

$y=x^{3}.$

(a) Find the differential  $dy$  of  $y=x^{3}$  at  $x=2$.

(b) Use differentials to find an approximate value for  $1.9^{3}$.

Foundations:
What is the differential  $dy$  of  $y=x^{2}$  at  $x=1?$
Since  $x=1,$  the differential is  $dy=2xdx=2dx.$

Solution:

(a)

Step 1:
First, we find the differential  $dy.$
Since  $y=x^{3},$  we have
$dy\,=\,3x^{2}\,dx.$

Step 2:
Now, we plug  $x=2$  into the differential from Step 1.
So, we get
$dy\,=\,3(2)^{2}\,dx\,=\,12\,dx.$

(b)

Step 1:
First, we find  $dx.$  We have  $dx=1.9-2=-0.1.$
Then, we plug this into the differential from part (a).
So, we have
$dy\,=\,12(-0.1)\,=\,-1.2.$

Step 2:
Now, we add the value for  Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle dy}   to  Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle 2^3}   to get an
approximate value of  Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle 1.9^3.}
Hence, we have
Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle 1.9^3\,\approx \, 2^3+-1.2\,=\,6.8.}

Final Answer:
(a)     Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle dy=12\,dx}
(b)     Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle 6.8}

Return to Sample Exam