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Factorising quadratics

Factorising quadratics. is in the form ax 2 + bx + c. So a = 6, b = 1, c = -2. 6x 2 + x - 2. You need to find 2 numbers which multiply to give ac (in this case -12) and add to give b (in this case +1). Now list the factors of ac. 1 -12. -1 12. 2 -6. -2 6. 3 -4. -3 4. -3

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Factorising quadratics

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  1. Factorising quadratics is in the form ax2 + bx + c So a = 6, b = 1, c = -2 6x2 + x - 2 You need to find 2 numbers which multiply to give ac (in this case -12) and add to give b (in this case +1) Now list the factors of ac

  2. 1 -12 -1 12 2 -6 -2 6 3 -4 -3 4 -3 4 Decide which pair adds up to b ie +1 Replace the x term by these two values ie -3x and 4x so 6x2 + x – 2 becomes 6x2 + 4x -3x -2 Now factorise it as a four term expression

  3. 6x2 + 4x -3x -2 = 2x (3x + 2) +1 (-3x - 2) There is nothing common in the second part so take out +1 Unfortunately, the signs in the second bracket are – those in the first bracket So instead of taking out +1 we take out -1 = 2x (3x + 2) -1 (3x + 2) The brackets are now the same so take them out as common factors So 6x2 + x - 2= ( 3x + 2 ) ( 2x -1 )

  4. 6x2 + 4x -3x -2 = 2x (3x + 2) +1 (-3x - 2) = 2x (3x + 2) -1 (3x + 2) So 6x2 + x - 2= ( 3x + 2 ) ( 2x -1 ) check F O I L 6x2 -3x +4x -2 = 6x2 + x - 2

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