Solve Quadratic Function: Find x-Intercepts & Just Touches x-Axis

[wellY--3]

1. A quadratic fraction is of the form f(x)= 2x^2+bx+5
Find the values for b where the graph of f(x):
1.just touches the x-axis
2. has two x-intercepts
3.does not cut or touch the x-axis


I've tried using the quadratic formula but i have no idea how to do it when there are two unknowns. For number one i thought it must be when y=0 and I've tried substituting other numbers in but other than that i can't start. Can you please start me off?

 

[FrogPad]

When you apply the quadatic formula what are you getting? If x_{12} = \frac{-b \pm \sqrt{b^2-4ac}}{2a} then what does it mean if x_{12} is complex? What about if x_{12} is real? What condition yields a x_{12} as complex? and which condition yields x_{12} as real?

 

[wellY--3]

b has to be a real number but how can you work it out when there are two unknowns .. x is unknown and so is b

 

[danago]

Using the quadratic formula FrogPad posted, pay special attention to the b^2-4ac part.

 

[cristo]

You do not need to look at the complete quadratic formula, just the discriminant b²-4ac. Do you know the condition on the discriminant for the equation to have (i)two real roots; (ii) one real repeated root; (iii) no real roots (equivalent to saying the equation has complex roots)?

 

[Moridin]

b has to be a real number but how can you work it out when there are two unknowns .. x is unknown and so is b

Yes, but that is before you take into account the 3 subtasks. If a graph touches the x-axis once, how does that affect the number of real solutions to the equation?

 

[wellY--3]

ooooooook so...
1. b=square root of 40
2 b is greater than square root of 40
3. b is less than square root of 40

is that right?