Undoing Addition
Part of the Fundamentals section of Coddy's Math journey. Lesson 6 of 64.
A one-step equation hides the unknown behind a single operation. In x + 7 = 12 the operation is "add 7". To free x, do the opposite to both sides: subtract 7.
x + 7 - 7 = 12 - 7
x = 5
Adding and subtracting are inverse operations: each undoes the other, so +7 followed by -7 leaves the number where it started. Every one-step equation is solved by spotting the operation done to the unknown and applying its inverse to both sides.
It does not matter which side the unknown is on: 9 = x + 4 becomes 5 = x by the same move, and that is x = 5 written the other way round.
Challenge
Solve the equation x + 9 = 20 for x.
Try it yourself
Solve for x
Test yourselfThis lesson includes a short quiz. Start the lesson to answer it and track your progress.
All lessons in Fundamentals
1What an Equation Is
The EquationFinding the UnknownThe Same to Both SidesSolved Means AloneRecap: First Equations4Two-Step Equations
Two Layers to UndoSubtract, Then DivideAdd, Then DivideA Division InsideNegatives in Two StepsRecap: Two-Step Equations2One-Step Equations
Undoing AdditionUndoing SubtractionUndoing MultiplicationUndoing DivisionInverse OperationsRecap: One-Step Equations5Tidying First
Like TermsTerms and Numbers MixedThe Distributive PropertyDistributing a SubtractionTidy, Then SolveRecap: Tidying First