Write and evaluate expressions with variables · lesson

Plug in the number

💡 To evaluate an expression, put the given number in place of the letter and do the math.

A letter like x is a variable. It stands for a number.

To evaluate, replace the letter with its value. Use parentheses so you don't lose a times sign.

A number next to a letter means multiply. So 4x means 4 times x.

🧠 Let's work it out

Find the value of 4x + 3 when x = 5.

What do you think comes next?

variable: a letter that stands for a number
evaluate: find the value by putting in the numbers
expression: a math phrase with numbers, letters, and operations, but no equals sign

💡 Do parentheses first, then exponents, then multiply and divide, then add and subtract.

Math has rules for what 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.

A fraction bar or a ÷ after parentheses means finish the top part first.

🧠 Let's work it out

Find the value of 3(k + 2) − k when k = 4.

What do you think comes next?

💡 Square the number first, then multiply by what is in front of it.

An exponent tells how many times to multiply a number by itself. b² means b × b.

In 2b², only the b is squared. Square b first, then multiply by 2.

🧠 Let's work it out

Find the value of 2b² + 3 when b = 4.

What do you think comes next?

💡 Turn each math word into a symbol, and watch the order for "less than".

Sum means add. Difference means subtract. Product or times means multiply. Twice means 2 times.

"5 less than t" means start with t and take away 5. So it is t − 5, not 5 − t.

"The sum of y and 2" is one group. If you multiply it, put it in parentheses.

🧠 Let's work it out

Write "3 times the sum of y and 2".

What do you think comes next?