Formulas

Hello, and welcome to Myriora Maths! A square has four equal sides. How long is the edge all the way round? There's a rule that works for every square. It's called a formula. Let's see.

Call the side s. The perimeter is four sides, so P equals four s. That's a formula: a rule written with letters.

To use it, put the number in place of the letter. If the side is seven centimetres, P is four times seven: twenty-eight centimetres. Putting a number in is called substituting.

Some formulas have two letters. The area of a rectangle is A equals l times w: the length times the width. If l is six and w is four, A is twenty-four.

Some formulas have two steps. A taxi costs two pounds, plus five pounds for every mile. So the cost is C equals five m add two. For three miles, do the times first: five times three is fifteen. Add two: seventeen pounds.

Let's try one together. P equals four s. What is P when s is nine? Say it out loud!

Four times nine is thirty-six. So P is thirty-six!

Here's a question that catches lots of people. y equals two x add three. What is y when x is four? Some people put the four next to the two, and get twenty-four add three: twenty-seven. But two x means two times x. Two times four is eight, add three: eleven!

Your turn! T equals ten add three m. What is T when m is five? Pick the right card. You've got twenty seconds, or pause the video. Go!

Time's up! Three m is three times five: fifteen. Ten add fifteen is twenty-five!

Quick review. A formula is a rule written with letters. To use it, substitute: put the numbers in place of the letters. A number next to a letter means times. And do the times before the add. See you next time!