Find all points where (4x^2)+(y^2)=8x+4y+4 has a horizontal or vertical tangent line. I graphed it and know that it has horizontal tangent lines at (1,0) and (1,-4) and vertical tangent lines at (0,-2) and (2,-2), I just need to show work.

- clara1223

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- DanJS

have you learned the chain rule , or implicit differentiation

- clara1223

yes I have, how would it apply here?

- DanJS

you want to find the derivative of y with respect to x dy/dx
that will tell you the slope of a tangent line to that curve at any point

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- clara1223

okay one second.

- clara1223

I get that y' is (-8x+8)/(2y+4)

- DanJS

umm

- clara1223

is that wrong?

- DanJS

8x + 2y * y' = 8 + 4y '

- DanJS

just was looking , i didnt calculate it

- clara1223

oh no! my original question is a typo, its supposed to be (4x^2)+(y^3)-8x+4y+4

- clara1223

there is no = in it

- DanJS

ha, it's fine, same process

- clara1223

so I get 8x+2y*y'-8+4y'=0 and then i isolate y'

- DanJS

yeah you just messed up a sign in the result,
move all terms with y ' to one side, the rest the other, factor out y', divide by the rest
y ' = (-4x + 4)/ (y - 2)

- clara1223

when you take the 2 out does it disappear?

- DanJS

yeah, just put it in simpler form, factor out 4 in the top and a 2 in the bottom, then simplify that fraction

- DanJS

that is the expression for the slope of a tangent to the relationship

- clara1223

you cant factor out a 2 in the bottom, unless you want to get y/2. so now i have 4(-x+1)/y-2

- DanJS

ha, not again, i did once when it was 2y-4 --- 2(y-2), the 2 divides into the 8 factored from the top

- clara1223

haha i realized that after i posted that. so once i have that how do i find the horizontal and vertical tangent lines?

- DanJS

do you know the slope of horizontal or vertical lines?

- clara1223

horizontal is 0 and vertical is undefined, right?

- DanJS

yep, find out when the derivative are those values

- clara1223

I'm confused, how would i do that?

- DanJS

y ' is the derivative you calculated aka the instantaneous slope of the graph at a point.
Set the expression y ' = 0 and solve where that occurs

- DanJS

y ' = 0 = (-4x + 4) / (y-2)

- clara1223

am I solving for x or y

- DanJS

just notice you have a fraction, a fraction is zero when the numerater is zero, and the fraction is undefined if the denominator is zero

- DanJS

solving, the derivative y ' = 0, when x=1
and
undefined at y = 2

- clara1223

so how does this get me the 4 points I need?

- DanJS

That tells you horizontal tangent(s) happen when x=1
vertical tangents(s) happen when y = 2

- DanJS

use x=1 in the original problem and calculate the y values. 2 possibly since it is a quadratic

- clara1223

and shouldnt it be y=-2?

- DanJS

the derivative y ' has y - 2 in the bottom, that was what you missed at first, just a sign

- clara1223

when I solved with y=-2 I get the correct points (the same points as when I graphed it with desmos)

- DanJS

##### 1 Attachment

- DanJS

THe tangent is horizontal when x = 1, solving for the y values , you get 2 of them from the quadratic..
\[(1 , ~[2\sqrt{3}+2])~~~~~and~~~~~(1,~ [-2\sqrt{3}+2])\]

- DanJS

The undefined value of y = 2 for the vertical tangent points, results in 2 x values from the problem because x is also a quadratic...

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