This key facts covers Sign Rules Cheat Sheet within Factorising for GCSE Mathematics. Revise Factorising in Algebra for GCSE Mathematics with 12 exam-style questions and 3 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 12 of 13 in this topic. Use this key facts to connect the idea to the wider topic before moving on to questions and flashcards.
Sign Rules Cheat Sheet
| Expression | Signs in Brackets |
|---|---|
| x² + bx + c (both positive) | (x + __)(x + __) |
| x² - bx + c (middle neg, end pos) | (x - __)(x - __) |
| x² + bx - c (end negative) | (x + __)(x - __) bigger number positive |
| x² - bx - c (end negative) | (x + __)(x - __) bigger number negative |
Q5 Factorise: x² + 7x + 10 [2 marks]
[1] 2 × 5 = 10, 2 + 5 = 7
[1] (x + 2)(x + 5)
Q6 Factorise: x² + x - 12 [2 marks]
[1] -3 × 4 = -12, -3 + 4 = 1
[1] (x - 3)(x + 4)
Q7 Factorise: x² - 8x + 12 [2 marks]
[1] -2 × -6 = 12, -2 + -6 = -8
[1] (x - 2)(x - 6)
Practice questions for Factorising
Which is the correct factorisation of 6x + 15?
A student factorises x² + 5x + 4 as (x + 4)(x + 4). Explain why this is incorrect and give the correct factorisation.
Quick recall flashcards
Factorising Rule
Find the HCF of all terms (numbers AND letters), put outside bracket, divide each term for inside.
Factorising
Take out highest common factor: 6x + 9 = 3(2x + 3)