Junior Cycle · Algebra ⏱ Tested in 3 exam questions since 2022 Lesson 14 of 21

JCHL Algebra 14: -b Formula

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You’ll learn

  • The minus b formula is on page 20 in your maths tables, and it solves any quadratic ax² + bx + c = 0
  • a = number in front of x², b = number in front of x, c = the constant
  • Use it when the quadratic will not factorise nicely (hints: "to 2 decimal places" or "in surd form")

Formula used

x = (-b ± √(b² - 4ac))/2a

On page 20 of the log tables — read it off, don’t memorise.

Most recent exam questions

  • 2025 Q13(c)
  • 2023 Q13(c)
  • 2022 Q9(d)
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