Pre-Algebra

The area of a triangle is half of its base times its height: A = 1/2 × b × h. The height is the straight-up distance from the base to the opposite corner, and it can fall outside the triangle.

Operations with decimals work like whole-number math, with one extra step: placing the decimal point. To add or subtract, line up the decimal points. To multiply, multiply as usual, then count the decimal places in both numbers. To divide by a decimal, move both decimal points to the right until the divisor is a whole…

Percentages are numbers out of 100. The symbol % means “per hundred,” so 25% means 25 out of 100, which is 0.25 or . To find a percent of a number, change the percent to a decimal and multiply. For example, 25% of 80 is . Before this lesson: You should know how to convert…

Proportions are equations that say two ratios are equal, like . To solve a proportion with a missing number, cross multiply. Multiply each top number by the bottom number on the other side, set the two products equal, then divide. In , you get , so . Before this lesson: You should know how to…

Fractions show part of a whole. The bottom number, the denominator, tells how many equal parts the whole is split into. The top number, the numerator, tells how many of those parts you have. Two fractions that name the same amount, like and , are called equivalent fractions. Before this lesson: You should know how…

The order of operations is the set of rules that tells you which part of a math problem to do first. Work inside parentheses first. Then do exponents. Next, multiply and divide from left to right. Last, add and subtract from left to right. Many students remember it as PEMDAS. Before this lesson: You should…

A square root of a number is a value that you multiply by itself to get that number. The square root of 25 is 5, because . We write this as . You can find exact square roots of perfect squares and estimate all the others. Before this lesson: You should know what squaring means…

Exponents are a short way to write repeated multiplication. The exponent tells how many times to multiply the base by itself. In , the base is 2 and the exponent is 4, so . You read it as “two to the fourth power.” Before this lesson: You should know how to multiply integers and how…

Absolute value is a number’s distance from zero on the number line. Distance is never negative, so an absolute value is always positive or zero. It is written with two straight bars. For example, and , because both numbers are 6 units from 0. Before this lesson: You should know how to find negative numbers…

Negative numbers are numbers less than zero. They are written with a minus sign, like −3 or −10. On a number line, negative numbers sit to the left of 0. The farther left a number is, the smaller it is, so . Before this lesson: You only need to know whole numbers (0, 1, 2,…