Math 1A Day 10
by EpicSkills32, Jul 16, 2014, 12:43 AM
![$\ [\text{Blog Post 103}] $](http://latex.artofproblemsolving.com/4/8/f/48f53d5f249965e567f1d247c776d9a32e023a79.png)
I really don't want to post because I want to be better than happysam, but oh well.
I can't say I really enjoyed this problem or anything, but when I stumbled upon the answer I thought it was pretty cool.
Let me know what you think.


![\[ y=\dfrac{e^x}{x} \]](http://latex.artofproblemsolving.com/f/1/e/f1efe129815f61083bcb6fd13bfa69c3d23995b0.png)
Point:

EDIT: solution
So this is a pretty straightforward derivative problem. The definition of a derivative is the slope of the tangent line to the curve, so yeah.
We first find the derivative of the function.
![\[ \dfrac{d}{dx} \left[\dfrac{e^x}{x}\right] = \,\text{?} \]](//latex.artofproblemsolving.com/f/a/e/fae923dac93a7bc4795a571f98049a74ebc65b39.png)
There might be some cool trick to do this, but Quotient Rule should suffice.
![\[ \dfrac{d}{dx}\left[ \dfrac{e^x}{x}\right] = \dfrac{(x)\left(\dfrac{d}{dx}e^x\right) - \left(e^x\right)\left(\dfrac{d}{dx}x\right) }{x^2} \]](//latex.artofproblemsolving.com/0/3/3/033b9c027577abf2140287ba0b1e2c9a4fd91f94.png)
![\[ = \dfrac{x\cdot e^x - e^x}{x^2} \]](//latex.artofproblemsolving.com/1/8/d/18d54f4888e59c30e8aa6da03d526d341e33dbcb.png)
We could simplify this by factoring out an
, resulting in
. However, this isn't necessary.
This is the general derivative for the function, but now we have to evaluate it at the
value to find the slope of the tangent line there. The point given is
.
Evaluating:
Yay we love exponents of
.
![\[ = \dfrac{e-e}{1} = 0 \]](//latex.artofproblemsolving.com/b/f/f/bff4fc69e2df7b8c2b3648c04c7b1ce4d067d26d.png)
But remember we're not done yet. This is only the slope of the tangent line. The problem asks for the equation of the tangent line.
So what do we have? We are given the point, and by calculating the derivative, we find the slope.
point+slope =......Point-Slope form!
By the way
We plug our given values in. Point:
Slope: 0
![\[ y-e = 0(x-1) \]](//latex.artofproblemsolving.com/2/4/9/249f474c10e59c3a882677b95ed6af376a6c8a01.png)
![\[ y-e = 0 \]](//latex.artofproblemsolving.com/b/2/d/b2d0119b22620e5ae80a074774357fa670b7a1d5.png)
![\[ y= e \]](//latex.artofproblemsolving.com/d/f/e/dfe2752efee4c299faad487511bcbeb6807f3a9b.png)
gg
We first find the derivative of the function.
![\[ \dfrac{d}{dx} \left[\dfrac{e^x}{x}\right] = \,\text{?} \]](http://latex.artofproblemsolving.com/f/a/e/fae923dac93a7bc4795a571f98049a74ebc65b39.png)
There might be some cool trick to do this, but Quotient Rule should suffice.
![\[ \dfrac{d}{dx}\left[ \dfrac{e^x}{x}\right] = \dfrac{(x)\left(\dfrac{d}{dx}e^x\right) - \left(e^x\right)\left(\dfrac{d}{dx}x\right) }{x^2} \]](http://latex.artofproblemsolving.com/0/3/3/033b9c027577abf2140287ba0b1e2c9a4fd91f94.png)
![\[ = \dfrac{x\cdot e^x - e^x}{x^2} \]](http://latex.artofproblemsolving.com/1/8/d/18d54f4888e59c30e8aa6da03d526d341e33dbcb.png)
We could simplify this by factoring out an


This is the general derivative for the function, but now we have to evaluate it at the


Evaluating:
\[ \dfrac{d}{dx} \left[\dfrac{e^x}{x}\right] \right|_{x=1} \, = \dfrac{e^{(1)} (1) -e^{(1)}}{(1)^2} \]
Yay we love exponents of

![\[ = \dfrac{e-e}{1} = 0 \]](http://latex.artofproblemsolving.com/b/f/f/bff4fc69e2df7b8c2b3648c04c7b1ce4d067d26d.png)
But remember we're not done yet. This is only the slope of the tangent line. The problem asks for the equation of the tangent line.
So what do we have? We are given the point, and by calculating the derivative, we find the slope.
point+slope =......Point-Slope form!
By the way
(This was a problem for me. I always used slope-intercept form or Ax+By=C, but then when I started doing calculus, I was really confused by how they found the equation of the line with just a point and the slope. Finally I was like "oh right, there's that other equation-of-a-line form" and I had to go back and learn that form. If you don't really know point-slope form like me, I suggest you go memorize it.
If you have a point
, and the slope
, then the equation of the straight line through that point with slope
is :
)
If you have a point




We plug our given values in. Point:

![\[ y-e = 0(x-1) \]](http://latex.artofproblemsolving.com/2/4/9/249f474c10e59c3a882677b95ed6af376a6c8a01.png)
![\[ y-e = 0 \]](http://latex.artofproblemsolving.com/b/2/d/b2d0119b22620e5ae80a074774357fa670b7a1d5.png)
![\[ y= e \]](http://latex.artofproblemsolving.com/d/f/e/dfe2752efee4c299faad487511bcbeb6807f3a9b.png)
gg
EDIT2: oops herp derp. No class on Fridays -> Day
This post has been edited 4 times. Last edited by EpicSkills32, Jul 16, 2014, 10:06 PM