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What is the sum of all numbers $a$ for which the graph of $y=x^2+a$ and the graph of $y=ax$ intersect one time?

 Jan 13, 2019
 #1
avatar+533 
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for this to be true, x^2+a and ax are equal at only one point. that means that x^2-ax+a's discriminant is 0, which means a^2-4a is 0. that means a is 0 or 4, and the sum is 4.

 

HOPE THIS HELPED!

 Jan 13, 2019
 #2
avatar+99585 
+1

y = x^2 + a  is a parabola

y = ax is a line

 

If the line intersects the parabola, it will be tangent to the parabola

The slope of a line tangent to the parabola at any point is 2x

So...the slope of the line = a = 2x

 

So....subbing in for a in both equations, we have that

 

y = x^2 + 2x

y = 2x (x)  = 2x^2

 

Setting these equal we have that

 

x^2 + 2x  = 2 x^2   rearranging, we have

 

x^2 - 2x  = 0      factor

 

x( x - 2) = 0     setting each factor to 0 and solving for x produces

 

x = 0      or x =   2

 

So....a =  2(0)  = 0     or  a = 2(2)  =  4

 

And the sum of these values for a  is 4

 

 

cool cool cool

 Jan 13, 2019
 #3
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0

thank you both!

Guest Jan 13, 2019

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