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What is the maximum value of c such that the graph of the parabola y = \dfrac{1}{3}x^2 has at most one point of intersection with the line y = x+c?

 Oct 31, 2014

Best Answer 

 #1
avatar+128475 
+8

Let y = (1/3)x^2  = g(x)

We are looking for the point where y = x + c  is tangent to g(x)

Beause the slope of y = x + c  = 1   we are looking for the point where the derivative of g(x) has a slope = 1.

So

g ' (x)  =  2/3x   ....set this = 1    and x = 3/2

And to find the y coordinate of this point we have  y= (1/3)(3/2)^2  = (1/3)(9/4) = 9/12 = 3/4

So, since the point (3/2, 3/4) is also on y = x + c    we have

3/4 = 3/2 + c      subtract 3/2 from both sides

c = -3/4

Here's the graph.....https://www.desmos.com/calculator/ugbqgcgqrf

 

 Oct 31, 2014
 #1
avatar+128475 
+8
Best Answer

Let y = (1/3)x^2  = g(x)

We are looking for the point where y = x + c  is tangent to g(x)

Beause the slope of y = x + c  = 1   we are looking for the point where the derivative of g(x) has a slope = 1.

So

g ' (x)  =  2/3x   ....set this = 1    and x = 3/2

And to find the y coordinate of this point we have  y= (1/3)(3/2)^2  = (1/3)(9/4) = 9/12 = 3/4

So, since the point (3/2, 3/4) is also on y = x + c    we have

3/4 = 3/2 + c      subtract 3/2 from both sides

c = -3/4

Here's the graph.....https://www.desmos.com/calculator/ugbqgcgqrf

 

CPhill Oct 31, 2014

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