Explanation:
Use the quadratic formula x = −b ± √(b2 − 4ac)2a when a quadratic equation ax2 + bx + c = 0 cannot be easily factorised, or when the question asks for solutions to a given number of decimal places or significant figures, or in surd form.
Always rearrange the equation so that the right-hand side equals 0 first, then write down the values of a, b, and c including their signs. Put negative numbers in brackets when substituting into (−b)2 − 4(a)(c) so your calculator squares the negative sign properly.
Example 1: Solve 2x2 − 7x − 3 = 0 to 2 d.p.
Solution:
Here a = 2, b = −7, c = −3
x = −(−7) ± √((−7)2 − 4(2)(−3))2(2)
x = 7 ± √(49 + 24)4 = 7 ± √734
x = 3.8860... or x = −0.3860...
Answer: x = 3.89 or x = −0.39 (2 d.p.)
Example 2: Solve x2 + 6x = 4 in the form p ± √q.
Solution:
Rearrange to x2 + 6x − 4 = 0 (a = 1, b = 6, c = −4)
x = −6 ± √(62 − 4(1)(−4))2(1) = −6 ± √522
Since √52 = √4 × √13 = 2√13:
x = −6 ± 2√132 = −3 ± √13
Answer: x = −3 ± √13