What is the vertex of y=—x^2 + 4x -1

Question

Hi, this equation makes no sense. When I calculate the vertex I get (2,11) but my book is saying the answer is (2,3)? I don't understand how it can be (2,3) . This is what I did: 

 

(-b)/(2a) = (-4)/(-2)= 2 

 

then I plugged the 2 in the equation and got 11. 

 

 

Answer 1

Yea, when you plugged x in, you did x^2 and got 4, but remember it says -x, so it's really going to be -4. You solved for y thinking that the x was negative before you squared it.

Answer 2

Hi Alex, to find the vertex (x, y), first find x, which is equal to -b/2a. Here b = 4 and a = -1.

 

Therefore, x = -4/(-2) = 2.

 

Now solve for y using the value of x, which is 2.

 

y = -(2)² + 4 * 2 - 1

 

y = -4 + 8 - 1

 

y = 3.

 

The vertex is: (x, y) = (2, 3)

 

Questions?