Solving Linear Inequalities (OCR GCSE Maths)

What is a linear inequality?

• An inequality tells you that one expression is greater than (“>”) or less than (“<”) another
  • “⩾” means “greater than or equal to”
  • “⩽” means “less than or equal to”
• A linear inequality only has constant terms (numbers with no letters) and terms in x (and/or a y); but no x² terms or terms with higher powers of x
  • For example, 3x² > 12 is not a linear inequality (it is a quadratic inequality)
• For example, 3x + 4 ⩾ 7 would be read “3x + 4 is greater than or equal to 7”. 

How do I solve linear inequalities?

• Solving linear inequalities is just like Solving Linear Equations
  • Follow the same rules, but keep the inequality sign throughout
  • If you change the inequality sign to an equals sign you are changing the meaning of the problem
• When you multiply or divide both sides by a negative number, you must flip the sign of the inequality 
  • e.g. 1 < 2 → [times both sides by (–1)] → –1 > –2 (sign flips)
• Never multiply or divide by a variable (x) as this could be positive or negative
• The safest way to rearrange is simply to add & subtract to move all the terms onto one side
• You also need to know how to use Number Lines, Set Notation and deal with “Double” Inequalities

How do I represent linear inequalities on a number line?

• Inequalities such as and can be represented on a normal number line using an open circle and an arrow
  • For , the arrow points to the left of 
  • For , the arrow points to the right of
• Inequalities such as and can be represented on a normal number line using a solid circle and an arrow
  • For , the arrow points to the left of 
  • For , the arrow points to the right of
• Inequalities such as and can be represented on a normal number line using two circles at and and a line between them
  • For or use an open circle
  • For or , use a solid circle
• Disjoint inequalities such as " or " can be represented with two circles at  and , an arrowed line pointing left from and an arrowed line pointing right from , and a blank space between and 

How do I represent linear inequalities using set notation?

• We use curly brackets and a colon in set notation. means "x is in the set ..."

• • For example; if x is greater than 3, then in set notation,

• However, if x is between two values, then the two end values must be written in separate sets, using the intersection symbol, 

• • For example, if x is greater than 3 and less than or equal to 5, then in set notation,

• Similarly, if x is less than one value or greater than another (disjoint), then the two end values must be written in separate sets using the union symbol, 

• • For example, if x is less than 3 or greater than or equal to 5, then in set notation,

How do I solve double inequalities?

• Inequalities such as can be solved by doing the same thing to all three parts of the inequality
  • Use the same rules as solving linear inequalities