The modulus function The modulus of x, written x and is defined by x = x if x 0 x = -x if x < 0. Sketch of y = x y x y = x y = x y = x y = - x Sketch the curve y = f(x), using a dashed line for points below the x-axis Reflect any part of the curve below the x-axis in the x-axis.
Sketch y = 2x - 1 The graph of y = 2x - 1 has the right-hand branch with equation y = 2x – 1. The left-hand branch is the negative of 2x – 1, i.e. y = -2x + 1. y x y = 2x - 1 y = -2x +1
Sketch y = (x-1)(x-2) x y
Sketch y = cos x x y = cos x
Equations involving modulus Solve the equation 2x - 1 = 3 y = 3 Left-hand branch: -2x + 1 = 3 x = - 1 Right-hand branch: 2x - 1 = 3 x = 2 Solution: x = -1 or x = 2
Examples Solve the equation 2x - 1 = x - 2 Critical values x 2 2x - 1 -(2x – 1) 2x – 1 2x - 1 x - 2 -(x – 2) -(x – 2) x - 2 x < ½ : -2x + 1 = -x + 2 x = - 1 ½ x 2 : 2x - 1 = -x + 2 x = 1 x > 2 : 2x - 1 = x - 2 x = -1 Not consistent. Solution: x = -1 or x = 1 Other methods: Square both sides or graphical method.
Examples Solve x = 2x + 1 Square both sides x 2 = (2x + 1) 2 x 2 = 4x 2 + 4x + 1 3x 2 + 4x + 1= 0 (3x + 1)(x + 1) = 0 x = -1/3 or x = - 1 Solve x-3 = 3x + 1 Square both sides (x – 3) 2 = (3x + 1) 2 x 2 - 6x+ 9 = 9x 2 + 6x + 1 8x x – 8 = 0 2x 2 + 3x – 2 = 0 (2x -1)(x + 2) = 0 x = - 2 or x = ½
Inequalities involving modulus x < a -a < x < a x > a x a Example x < 2 -2 < x < x > 2 x
Examples Solve the inequality 2x - 1 < 3 y = 3 2x - 1 < 3 - 3 < 2x – 1 < 3 -2 < 2x < 4 -1 < x < 2
Examples Solve the inequality 2x - 3 5 y = 5 2x - 5 5 2x – 3 -5 or 2x – 3 5 x -1 or x 4
Examples Solve the inequality x + 2 < 3x + 1 Square both sides: (x + 2) 2 < (3x + 1) 2 x 2 + 4x + 4 < 9x 2 + 6x + 1 8x 2 + 2x - 3 > 0 (4x + 3)(2x – 1) > 0 x ½ y x
Examples Solve the inequality x + 2 > 2x + 1 Critical values x < -2 x - 2 x + 2 -x - 2 x + 2 2x + 1x + 1 2x +1 when x 2x + 1 x < - 1 when x - 2: x + 2 > 2x + 1 x < 1 x y Solution x < 1
Example A graph has equation y = 2x + x + 2 . Express y as a linear functions of x ( y = mx + c). When x < -2 : y = 2x – x – 2 = x – 2 y = x - 2 When x -2 : y = 2x + x + 2 = 3x + 2 y = 3x + 2