How do I know when a quadratic function crosses the y-axis?

The key thing to remember here is that the y-axis is a straight line and all of the points that lie on it have the x-value of 0. This means that the curve crosses the y-axis when x=0 so you can substitute this into the function to calculate the y value.

For example, if y = x2 + 6x + 8, we would substitute x with 0 to calculate y.

y = x2 + 6x + 8

y = 02 + 6(0) + 8

y = 0 + 0 + 8 = 8

Therefore the function crosses the y-axis when x is 0 and y is 8, the co-ordinate (0,8).

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