Solving quadratics by factorising
Objective. Solve x² + bx + c = 0 by factorising into two brackets.
- 1. Learn
- 2. Worked example
- 3. Practice
- 4. Feedback
- 5. Continue
Learn: the key idea
Find two numbers that multiply to give c and add to give b. If a product is zero, at least one factor must be zero.
Worked example
Solve x² + 2x − 15 = 0.
- 1Two numbers multiplying to −15 and adding to 2 are 5 and −3.
- 2(x + 5)(x − 3) = 0.
- 3So x + 5 = 0 or x − 3 = 0.
Answer: x = −5 or x = 3
Concept mastery
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Start practisingCommon mistake. Forgetting the negative root when square rooting.
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