Education.com
Try
Brainzy
Try
Plus

# Tip #39 to Get a Top ACT Math Score (page 2)

By McGraw-Hill Professional
Updated on Sep 7, 2011

1. A The question describes a circle. That's what a circle is, it's all the points in a plane that are a certain distance from a center point. Remember that radius is the distance from the center to the circle, and the diameter is twice that distance, i.e., twice the radius.
2. H The center is (h, k) based on the equation (xh)2 + (yk)2 = r2. So match up the given equation to (xh)2 + (yk)2 = r2, and determine h and k. For (x – 8)2 + (y + 2)2 = 49, h = 8 and k = –2. Notice that we have (y + 2) but the formula is for (yk), so we switch the sign of 2 to get –2. So, the center is (8, –2).
3. D We just need to apply the info we are given into the equation for a circle, which is (xh)2 + (yk)2 = r2. Since the center is (5, –3) and the radius is 6, h = 5, k = –3, and r = 6. So D is the answer. Notice that we can easily get this question with process of elimination. For example, since r = 6, the number to the right of the equals sign, r2, must equal 36, not 6.
4. J When a picture is described but not shown, draw it. The circle has center (1, 4). Since it is tangent to the y axis, our sketch shows that it has a radius of 1. So h = 1, k = 4, and r = 1. Choice J works.
5. D When a picture is described but not shown, draw it. The sketch shows that the center must be (3, 3) and that the radius is therefore 3. So h = 3, k = 3, and r = 3. Choice D works. Watch out for choice B, which looks okay, but has a minus sign instead of a plus sign between the parentheses! That's why you've got to cut out the flash cards at the end of this book and memorize the equation for a circle.

Go to: Tip #40

## Special Editions

### Browse by Topic

Tuition-free online school for Washington students.