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Math · 9th Grade
Writing a Geometry Proof
The structure, and the habit that makes proofs writable rather than mysterious.
How to use it
Work backwards from what you must prove. Ask what would be enough to conclude it.
Free · print-ready PDF · US Letter and A4
- Grade
- 9th Grade
- Ages
- 14–15
- Subject
- Math
- Pages
- 1
The rest of the 9th Grade wall
49 more posters, all free to print — 11 of them here.
Function Notationf(x) is not multiplication — and everything else follows from getting that right.Domain and RangeWhich inputs are allowed, and which outputs actually appear.Factoring Out the GCFThe first thing to try, every single time, before anything else.Factoring TrinomialsTwo numbers that multiply to c and add to b — and what to do when a is not 1.Difference of SquaresThe pattern worth spotting instantly, and the one that looks similar but does not factor.The Quadratic FormulaThe method that always works, and the sign errors that ruin it.The DiscriminantOne calculation that tells you how many solutions there are before you find them.Anatomy of a ParabolaVertex, axis, roots and direction — everything a quadratic graph tells you.Choosing a Systems MethodGraphing, substitution or elimination — decided by the shape of the equations.Arithmetic SequencesAdd the same amount each time — and the formula that jumps straight to any term.Proving Triangles CongruentFive shortcuts that work, and the two that look like they should but do not.