Undoing Addition
Part of the Fundamentals section of Coddy's Algebra journey. Lesson 6 of 64.
A one-step equation has one operation done to the unknown. In x + 7 = 12 the operation is "add 7". To get x alone, do the opposite to both sides: subtract 7.
x + 7 - 7 = 12 - 7
x = 5
Adding and subtracting are inverse operations. Each one undoes the other. If you add 7 to a number and then subtract 7, you get the number you started with. To solve a one-step equation, find the operation done to the unknown and apply its inverse to both sides.
The unknown can be on either side. Take 9 = x + 4. Subtract 4 from both sides to get 5 = x. The equation 5 = x is x = 5 written the other way round.
Challenge
EasySolve 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