The short version A linear pair is two angles that sit side by side and share a vertex and a side, with their other two sides forming a straight line. The two angles in a linear pair always add up to 180°. To find a missing one, subtract from 180: if one angle is 47°,…
The short version A scalene triangle has three sides of different lengths. Because the sides are all different, the three angles are all different too. Scalene triangles can be acute, right or obtuse. A 3-4-5 right triangle is scalene. If two angles match, the triangle is isosceles, not scalene. Checking the angles is often faster…
The short version Nobody invented math. It was built up over tens of thousands of years by people in many parts of the world, starting with counting and tally marks. The oldest written math we have comes from Mesopotamia and Egypt, roughly 5,000 to 3,500 years ago. Big pieces came from different places: proof-based geometry…
The short version To multiply decimals, ignore the decimal points, multiply the numbers as if they were whole numbers, then put the decimal point back. Count the digits after the decimal point in both factors and add them. The answer gets that many decimal places. You don’t line up the decimal points when you multiply….
The short version Complementary angles are two angles whose measures add up to 90°. To find the complement of an angle, subtract it from 90. The complement of 35° is 90 – 35 = 55°. The two angles don’t have to touch. They only have to add to 90°. Only acute angles have complements. A…
The short version A number line is a straight line with numbers marked at equal spaces. Numbers get bigger as you move right and smaller as you move left. Zero sits in the middle. Positive numbers go to the right, negative numbers go to the left, and the arrows mean it keeps going forever. Adding…
The short version An obtuse angle is any angle greater than 90° and less than 180°. It’s wider than the corner of a piece of paper but not flat. Exactly 90° is a right angle and exactly 180° is a straight angle. Neither one is obtuse. When you measure an obtuse angle with a protractor,…
The short version Inverse operations are pairs of operations that undo each other. Addition and subtraction are one pair. Multiplication and division are the other main pair. Kids use them to check work: if 72 – 38 = 34, then 34 + 38 should equal 72. They’re how you solve equations. To get x alone…
The short version The mode in math is the value that shows up most often in a set of data. In 2, 3, 4, 7, 7, 7, 9, the mode is 7. To find it, put the numbers in order, count how many times each appears and pick the one with the highest count. A…
The short version An undefined slope belongs to a vertical line. The line goes straight up and down, so it never moves left or right. Slope is rise ÷ run. On a vertical line the run is 0, and you can’t divide by 0, so the slope has no value at all. Quick test with…