#4
Top 50 Mistake
Forgetting to change the sign when moving terms
"The Sign Slip"
The Mistake in Action
Solve $x + 7 = 12$
Wrong: $x = 12 + 7$ $x = 19$
Why It Happens
Students "move" the 7 to the other side but forget that moving involves the inverse operation. They keep the + sign instead of changing it to −.
The Fix
Don't think of "moving" terms — think of doing the inverse operation to both sides.
To remove "+7" from the left side, subtract 7 from both sides: $$x + 7 = 12$$ $$x + 7 - 7 = 12 - 7$$ $$x = 5$$
Memory aid: When a term "crosses the bridge" (equals sign), it changes its sign.
Spot the Mistake
Can you identify where this student went wrong?
Solve $x + 7 = 12$
$x = 12 + 7$
$x = 19$
Click on the line that contains the error.
Related Topics
Learn more about the underlying maths: