4.1 Simultaneous Equations


4.1 Simultaneous Equations

(A) Steps in solving simultaneous equations:
  1. For the linear equation, arrange so that one of the unknown becomes the subject of the equation.
  2. Substitute the linear equation into the non-linear equation.
  3. Simplify and expressed the equation in the general form of quadratic equation a x 2 + b x + c = 0
  4. Solve the quadratic equation. 
  5. Find the value of the second unknown by substituting the value obtained into the linear equation.

Example:
Solve the following simultaneous equations.
y + x = 9 x y = 20

Solution:
For the linear equation, arrange so that one of the unknown becomes the subject of the equation.
y + x = 9 y = 9 x

Substitute the linear equation into the non-linear equation.
x y = 20 x ( 9 x ) = 20 9 x x 2 = 20

Simplify and expressed the equation in the general form of quadratic equation a x 2 + b x + c = 0
9 x x 2 = 20 x 2 9 x + 20 = 0

Solve the quadratic equation. 
x 2 9 x + 20 = 0 ( x 4 ) ( x 5 ) = 0 x = 4  or  x = 5

Find the value of the second unknown by substituting the value obtained into the linear equation.
When  x = 4 , y = 9 x = 9 4 = 5 When  x = 5 , y = 9 x = 9 5 = 4