Mathematics for IGCSE & O level - Equations (Section 6 - No. 8)
When using substitution to solve simultaneous equations, what should you always substitute?
The expression for the subject into the other equation.
The value of x into both equations.
The value of y into the first equation only.
The constant term into both equations.
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Always substitute the expression you have found for one variable into the equation containing the other variable.