# Finding value to make two graphs tangent

• Nov 19th 2011, 06:55 AM
benny92000
Finding value to make two graphs tangent
Find a negative value of k so that the graph of y = x2 - 2x + 7 and the graph of y = kx + 5 are tangent?

It says the answer is -2-2sqt2

I graphed this and it didn't look tangent to me. Thanks.
• Nov 19th 2011, 07:05 AM
emakarov
Re: Finding value to make two graphs tangent
It does look tangent.

Hint: consider the difference of the two functions and find when it has a single root.
• Nov 19th 2011, 07:19 AM
benny92000
Re: Finding value to make two graphs tangent
I am putting it in the calculator wrongly. When I evalued K and then put it in the calculator, it looked tangent.

This is how I am putting it in: "(-2-2sqt2)x+5"
• Nov 19th 2011, 08:11 PM
mr fantastic
Re: Finding value to make two graphs tangent
Quote:

Originally Posted by benny92000
I am putting it in the calculator wrongly. When I evalued K and then put it in the calculator, it looked tangent.

This is how I am putting it in: "(-2-2sqt2)x+5"

You need the line and the parabola to intersect at a single point. Therefore you need the discriminant of $x^2 - 2x + 7 - (kx + 5) = 0$ to equal zero ....
• Nov 21st 2011, 02:12 PM
benny92000
Re: Finding value to make two graphs tangent
Ok so I got x^(2) (-2-k)x + 2 = 0

Ultimately I got k^(2) +4k -4 = 0 I used quadratic formula and came up with the correct answer. So theoretically, when you subtract the two equations, the solution(s) of that equation is the intersection(s) of both equations? And with resepect to the calculator, did I plug it on wrongly? Why did they not appear to intersect?
• Nov 21st 2011, 02:54 PM
emakarov
Re: Finding value to make two graphs tangent
Quote:

Originally Posted by benny92000
So theoretically, when you subtract the two equations, the solution(s) of that equation is the intersection(s) of both equations?

I would be careful with terminology. First, one can only talk about the intersection of graphs (which are geometrical figures), not equations. Second, we can draw all points satisfying two equations $f_1(x,y)=0$ and $f_2(x,y)=0$, but this would not be the set of points satisfying $f_1(x,y)-f_2(x,y)=0$. For example, points satisfying 2x + 3y = 0 and x + y = 0 are lines that intersect at just one point (0, 0), but points satisfying x + 2y = 0 fill a whole line. What we have in the original problem are equations $y = x^2 - 2x + 7$ and $y = kx + 5$, to be sure, but not arbitrary equations. Rather, they are definitions of functions with only y in the left-hand side.

So, the correct statement is, If you have functions $y=f_1(x)$ and $y=f_2(x)$, then the solutions of the equation $f_1(x)-f_2(x)=0$ give the (x-coordinates of the) intersection points of the graphs of the two functions.

Quote:

Originally Posted by benny92000
And with resepect to the calculator, did I plug it on wrongly? Why did they not appear to intersect?

I don't have a graphing calculator, so I am not sure. There are many possible reasons. E.g., you may need to adjust the window size, write "2*sqt(2)" instead of "2sqt2," etc. Try using 1.41 instead of sqt2.