December 4th, 2009, 07:42 AM  #1 
Senior Member Joined: Sep 2008 Posts: 199 Thanks: 0  tangent
Prove that for all values of m, the line $\displaystyle y=mx+\frac{a}{m}$ is a tangent to the parabola $\displaystyle y^2=4ax.$ My attempt: $\displaystyle y=2a^{\frac{1}{2}}x^{\frac{1}{2}}$ $\displaystyle \frac{dy}{dx}=\sqrt{\frac{a}{x}}$ $\displaystyle m=\sqrt{\frac{a}{x}}$ $\displaystyle a=m^2x$ $\displaystyle y=mx+\frac{m^2x}{m}$ $\displaystyle y=2mx$ I don't know how this is gonna help in my proof .. Last edited by skipjack; February 28th, 2018 at 02:01 PM. 
December 4th, 2009, 09:36 AM  #2 
Senior Member Joined: Feb 2009 From: Adelaide, Australia Posts: 1,519 Thanks: 3  Re: tangent
Substitute $\displaystyle y=mx+\frac{a}{m}$ into $\displaystyle y^2=4ax$. You will get a quadratic in x. How many solutions does it have? Solve for x and show that the parabola has slope m at the point of intersection.
Last edited by skipjack; February 28th, 2018 at 01:49 PM. 
December 4th, 2009, 11:59 AM  #3  
Math Team Joined: Dec 2006 From: Lexington, MA Posts: 3,267 Thanks: 408  Re: tangent Hello, mikeportnoy! Quote:
aswoods has the best idea . . . Find the intersection(s) of the line and the parabola. $\displaystyle \text{Substitute }y \:=\:mx + \frac{a}{m}\text{ into: }\:y^2 \:=\:4ax$ $\displaystyle \text{We have: }\;\left(mx + \frac{a}{m}\right)^2 \:=\:4ax \qquad\Rightarrow\qquad m^2x^2 \,+\, 2ax \,+\, \frac{a^2}{m^2} \:=\:4ax \qquad\Rightarrow\qquad m^2x^2 \,\, 2ax \,+\, \frac{a^2}{m^2} \:=\:0$ $\displaystyle \text{Multiply by }m^2:\;\;m^4x^2 \,\, 2am^2x \,+\, a^2 \:=\:0$ $\displaystyle \text{Factor: }\;(m^2x \,\, a)^2 \:=\:0$ $\displaystyle \text{Then: }\;m^2x \,\,a \:=\:0 \qquad\Rightarrow\qquad x \:=\:\frac{a}{m^2}$ $\displaystyle \text{The line and the parabola intersect exactly }once.$ . . $\displaystyle \text{Therefore, the line is }tangent\text{ to the parabola.}$ Last edited by skipjack; February 28th, 2018 at 01:57 PM.  
December 4th, 2009, 05:59 PM  #4 
Senior Member Joined: Sep 2008 Posts: 199 Thanks: 0  Re: tangent
thank you

December 4th, 2009, 10:15 PM  #5 
Global Moderator Joined: Dec 2006 Posts: 20,916 Thanks: 2199 
For t$\neq$0, the tangent through the point (atÂ², 2at) on the parabola has slope 1/t, so its equation is y = (1/t)x + at, i.e., y = mx + a/m, where m = 1/t. For m = 0, however, y = mx + a/m doesn't define a line, so it isn't the case that the given equation is that of a tangent to the parabola for all values of m.
Last edited by skipjack; February 28th, 2018 at 01:58 PM. 
December 5th, 2009, 12:13 AM  #6  
Senior Member Joined: Sep 2008 Posts: 199 Thanks: 0  Re: Quote:
True , I didn't realize that until you tell me. So it would be valid for all m, except for m=0. Last edited by skipjack; February 28th, 2018 at 02:00 PM.  
December 5th, 2009, 02:01 PM  #7 
Global Moderator Joined: Dec 2006 Posts: 20,916 Thanks: 2199 
Correct.


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