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# Online Math Dictionary: A

**Acceleration Acceleration is the change in something’s speed (velocity). When you are speeding up, you are accelerating. When you are slowing down, you are “accelerating” – well, that’s what they’d say in a Physics class, but we say “decelerating .”**

**Acre An acre is a unit of measure used for measuring land in the United States. An acre is 43,560 square feet or 4,840 square yards.**

**Alternate interior angles When a pair of parallel lines is intersected by a transversal (like in the diagram on the right), angles C and F are a set of alternate interior angles and D and E are a set of alternate interior angles.**

**In a drawing like this (where there is a set of parallel lines intersected by a transversal ), alternate interior angles are congruent (the same size.)**

**Alternating Sequence An alternating sequence is a list of numbers that goes positive. negative. positive. negative. or negative. positive. negative. positive .Example: -1. 2. -3. 4. -5. 6. -7. 8. For more info on sequences, check out my “What’s a sequence? ” lesson.**

**Amplitude The amplitude of a graph or a curve is kind of the height of the graph. You take the height above the x-axis ( 4 in the pic on the right) and the height below the x-axis ( 3 in the pic) and add them together to get the amplitude (which is 4 + 3 = 7 in the pic).**

**Annuity Mathematically speaking, an annuity is a series of equal payments made over a specified period of time. For example, if you put a certain amount of money each month into a savings account, that would be an annuity. Mortgages, monthly health insurance payments, and pension payments are other types of annuities.**

In the real world, an annuity is an investment (that you purchase) where you pay an annuity provider upfront to receive a certain amount of money each year or each month for a specified period. For example, when you retire, you can purchase a life annuity that pays out $1,000 each month for the rest of your life.

**Arc An arc of a circle is a segment of the circumference of the circle .**

**Area Wow, you wouldn’t believe how confusing the usual definitions for “area” in math dictionaries are! And it’s such a simple thing. So, here’s my explanation: If you are going to carpet a room, the amount of carpet you need IS the area. In the picture on the right, the area is the blue part .For more info on areas, check out my reference page on area formulas .**

**Arithmetic Working with numbers, arithmetic is the process and study of addition, subtraction, multiplication and division.**

**Associative Property of Addition The associative property of addition is a math rule that is always true. Here it is with letters: a + ( b + c ) = ( a + b ) + c Remember that you always do what’s in the parenthesis ( ) first. This rule just says that, when you are doing addition, it doesn’t matter which numbers you add first. You can add the a and b first OR you can add the b and c first and you’ll get the same answer. Here it is with numbers so you can check this for yourself! 2 + ( 3 + 4 ) = ( 2 + 3 ) + 4 NOTE: This does not work with subtraction!**

**Associative Property of Multiplication The associative property of multiplication is a math rule that is always true. Here it is with letters: a x ( b x c ) = ( a x b ) x c Remember that you always do what’s in the parenthesis ( ) first. This rule just says that, when you are doing multiplication, it doesn’t matter which numbers you multiply first. You can multiply the a and b first OR you can multiply the b and c first and you’ll get the same answer. Here it is with numbers so you can check this for yourself! 2 x ( 3 x 4 ) = ( 2 x 3 ) x 4 NOTE: This does not work with division!**

**Average This is the same thing as the “mean ” of a group of numbers. It’s weird to define “average” in words, but really easy to show what it is. Look at this list of numbers: 3, 6, 13, 4, 9. Add them up and we get 35. Now, divide by how many numbers there are. There are five numbers, so divide 35 by 5. We get 7 and that’s the average of this list of numbers. Notice that some of the numbers are above 7 and some are below.**

**Axiom An axiom is a rule in math that is always true. In fact, it is SO obviously true that you don’t have to prove it’s true. This is an axiom: The sun rises in the morning. You know this is true and I don’t have to prove it. This is not an axiom: My cat is gold. Yes, this is true, but how can you believe me without my proving it with a picture? Here’s proof! **