Brainlag PlusSolve any quadratic with the formula, round sensibly, and use b² − 4ac to count the solutions before you start.
When it will not factorise
Most quadratics do not factorise with whole numbers.
The formula always works:
x = (−b ± √(b² − 4ac)) ÷ 2a
- 2x² + 3x − 4 = 0: a = 2, b = 3, c = −4
- b² − 4ac = 9 − 4 × 2 × (−4) = 9 + 32 = 41
- x = (−3 ± √41) ÷ 4
- x = 0.85 or x = −2.35 (2 d.p.)
Write a, b and c down first, with their signs. Put negative numbers in brackets on the calculator: (−4), not −4 typed after a minus.
The rest of this lesson and its 7 practice questions are in Plus.
Screen 1 of 4, free to read.
- 7 exam-style questions, marked as you go, each with a worked solution
- The Equation Solver beside every question
- Anything you get wrong is saved to your flashcards