Using a Formula

A formula is an equation that is true for every case. Its letters stand for the quantities that change from one case to the next. The perimeter of a rectangle is $P = 2l+2w$. The distance travelled at a steady speed is $d = st$. The area of a rectangle is $A = lw$.

To use a formula, substitute the values you know and work out the one you want. Take a rectangle with length 5 and width 3. Put $l = 5$ and $w = 3$ into the perimeter formula: $P = 2\cdot 5+2\cdot 3 = 16$. Once the letters are replaced by numbers, the arithmetic gives $P$.