Math 7 · Unit 4, Lesson 2: Combining Like Terms · Tile 3 of 3
Subtracting Expressions
Take away every piece
Take away every piece
Subtracting an expression means taking away the whole group, every piece of it. Lesson 1, Tile 2 showed the trick: a minus in front of parentheses is an invisible negative one. Hand the negative one to every term inside, so every sign flips. Then combine.
Wrong
Only the first sign flipped.
Right
Every sign in the group flipped.
The −3 is taken away too, and taking away a debt adds.
Elle and Finn
To subtract a linear expression, add its opposite: change the sign of every term in the expression being subtracted.
Flip every sign in the second group, not just the first one. Then it's an adding problem.
Elle and Finn talk it over
I subtracted the first term and stopped. The rest can stay.
That's like returning one shoe and keeping the other.
Fair. Nobody needs one shoe.
So take back the whole pair. Every term.
When you subtract a group, every term in it changes sign.
A real one
Store A charges 5x + 12 dollars for x T-shirts with a design fee. Store B charges 3x + 15 dollars. How much more does Store A charge?
Store A charges 2x − 3 dollars more.
NC standard: NC.7.EE.1.