Finding only one variable
"The Halfway Stop"
The Mistake in Action
Solve: $x + y = 7$ and $x - y = 3$
Wrong: Add equations: $2x = 10$, so $x = 5$ Answer: $x = 5$
Why It Happens
Students successfully find one variable but forget to substitute back to find the other. They think the job is done after finding $x$.
The Fix
Simultaneous equations have two unknowns — you must find both.
After finding $x = 5$, substitute into either original equation: $$5 + y = 7$$ $$y = 2$$
Complete answer: $x = 5, y = 2$
Always check: "Have I found values for ALL the unknowns?"
Spot the Mistake
Can you identify where this student went wrong?
Add equations: $2x = 10$, so $x = 5$
Answer: $x = 5$
Click on the line that contains the error.
Related Topics
Learn more about the underlying maths: