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AQA GCSE · Question 16 · Algebra

Solve (x + 2)(x - 5) = 6x

How to approach this question

1. First, expand the brackets on the left-hand side of the equation. 2. Rearrange the equation so that all terms are on one side, equal to zero. This will give you a standard quadratic equation in the form ax² + bx + c = 0. 3. Solve the resulting quadratic equation. You can do this by either factorising, completing the square, or using the quadratic formula. 4. Remember that a quadratic equation usually has two solutions.

Full Answer

**Step 1: Expand the brackets.** (x + 2)(x - 5) = x(x - 5) + 2(x - 5) = x² - 5x + 2x - 10 = x² - 3x - 10 **Step 2: Rearrange the equation into the standard quadratic form ax² + bx + c = 0.** The equation is now: x² - 3x - 10 = 6x Subtract 6x from both sides to make the right side equal to zero: x² - 3x - 6x - 10 = 0 x² - 9x - 10 = 0 **Step 3: Solve the quadratic equation.** We can solve this by factorising or using the quadratic formula. *Method 1: Factorising* We need two numbers that multiply to make -10 and add to make -9. The numbers are -10 and +1. So, we can factorise the equation as: (x - 10)(x + 1) = 0 For the product to be zero, one of the factors must be zero. Either x - 10 = 0 => x = 10 Or x + 1 = 0 => x = -1 *Method 2: Quadratic Formula* For x² - 9x - 10 = 0, we have a=1, b=-9, c=-10. x = [-b ± √(b² - 4ac)] / 2a x = [ -(-9) ± √((-9)² - 4(1)(-10)) ] / (2 * 1) x = [ 9 ± √(81 + 40) ] / 2 x = [ 9 ± √121 ] / 2 x = [ 9 ± 11 ] / 2 So, x = (9 + 11) / 2 = 20 / 2 = 10 Or, x = (9 - 11) / 2 = -2 / 2 = -1 **Answer: x = 10 and x = -1**
The given equation is (x + 2)(x - 5) = 6x. **1. Expand the brackets:** We use the FOIL method (First, Outer, Inner, Last): (x * x) + (x * -5) + (2 * x) + (2 * -5) = x² - 5x + 2x - 10 Combine the x terms: = x² - 3x - 10 So the equation is now: x² - 3x - 10 = 6x **2. Rearrange to standard quadratic form (ax² + bx + c = 0):** Subtract 6x from both sides: x² - 3x - 6x - 10 = 0 x² - 9x - 10 = 0 **3. Solve the quadratic equation:** We can factorise this expression. We are looking for two numbers that multiply to give -10 and add to give -9. These numbers are -10 and +1. So, the equation becomes: (x - 10)(x + 1) = 0 This gives two possible solutions: - If x - 10 = 0, then x = 10. - If x + 1 = 0, then x = -1. The solutions are x = 10 and x = -1.

Common mistakes

✗ Errors in expanding the brackets, especially with negative signs. ✗ Incorrectly rearranging the equation (e.g., adding 6x instead of subtracting). ✗ Errors in factorising or in applying the quadratic formula. ✗ Finding only one of the two solutions.

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