Cambridge IGCSE Additional Mathematics · Syllabus 0606 · Simultaneous Equations
Excluded Value
What is Excluded Value?
A value of an unknown for which one of the original equations is undefined, most commonly a value that would make a denominator zero. Excluded values must be written down before the denominators are cleared, because multiplying through produces a polynomial equation that is defined at those values even though the original system is not; any candidate solution that lands on an excluded value is therefore an artefact of the clearing step and must be rejected rather than reported.
This definition is part of the Simultaneous Equations chapter in Cambridge IGCSE Additional Mathematics.
Common mistakes with Excluded Value
- “\((x-2)(x+7)=0\), and \(x=2\) looks right, so \(x=2\).” WHY IT FAILS A root is discarded only when something in the question forbids it — a stated restriction, an excluded value, or a context such as a length. “It looks nicer” is not one of those. FIX Carry both roots to the end. If one must go, name the condition that removes it in writing.