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Tip #38 to Get a Top ACT Math Score (page 2)

By McGraw-Hill Professional
Updated on Sep 7, 2011

1. D When the equation of a parabola is given in the form y = ax2 + bx + c, the c is the y-intercept. So for y = x2 + 2x –3, –3 is the y intercept, which has coordinates (0, –3), since the x value is 0 and the y value is –3.
2. G Rewrite y – 2 = – (x – 3)2 so it matches the form y = (xh)2 + k. Just add 2 to both sides to get y = – (x + 3)2 + 2. In this form, the vertex or minimum/maximum point is (h, k), so (–3, 2) is the maximum point.
3. E Use the graph. When y = 0, the graph is at two different x values, one positive and one negative.
4. F Our Skill makes this "hard" question easy! For the graph shown, the vertex clearly has a positive x and a positive y value, and the graph opens down. So we need h and k to be positive, and we need a negative sign in front of (x – 2)2. Answer choice F is the only equation that fits these requirements, y – 3 = – (x – 2)2.
5. B Our Skill makes this "hard" question easy! From the graph, we can see that k must be positive, since the graph opens up. And we can see that r must be negative, since there is a negative y intercept. The product of k and r is (positive)(negative) = negative.

Go to: Tip #39