Question

Find an equation of a line whose graph intersects the graph of the parabola y=x^2 at (a) two points, (b) one point, and (c) no point. (There is more than one correct answer for each.)

Answer 1

1) 2 points: 
We need to come up with a function that intersects the graph at two points, meaning has two (x,y) in common with the function. If you look at the graph of y=x^2, you see that it would be quite easy to draw a line that intersects the graph twice. In fact, there are an infinite number of functions that would satisfy this. 
One easy function is y=2. This is a horizontal line in which y=2 for all values of x. In the graph y=x^2, y=2 intersects twice.
2=x^2
x^2= √2 or -√2
the shared points are (√2,2) and (-√2,2)

b) one point:
Here, we want to find an equation with only one (x,y) in common with y=x². This is a bit trickier.
One easy solution is y=-x²
Looking at a graph of the two functions, you see that y=-x² is a reflection across the x-axis of y= x². The two functions have only one point in common: (0,0). 

c) no point in common
Take another look at the graph of y=x². You see that the function never crosses the x-axis. A simple function that will never intersect the graph is y=-2. Since y is negative for all values of x, it is guaranteed to never intersect y=x², a function in which y is positive for all negative or positive values of x.