# Cramer's Rule to Solve Systems of Equations

This is a tutorial on proving Cramer's rule of solving 2 by 2 systems of linear equations. Rules for 3 by 3 systems of equations are also presented. To check answers when solving 2 by 2 and 3 by 3 systems of equations, you may want to use these online Systems of Equations Calculator and Solver. .

 To find rules (or formulas) that may be used solve any system of equations, we need to solve the general system of the form a 1 x + b 1 y = c 1    (1) a 2 x + b 2 y = c 2    (2) We multiply equation (1) by b 2 and equation (2) by -b 1 and add the right and left hand terms. b 2 a 1 x + b 2 b 1 y = b 2 c 1    (1) - b 1 a 2 x - b 1 b 2 y = - b 1 c 2    (2) ____________________________ b 2 a 1 x - b 1 a 2 x = b 2 c 1 - b 1 c 2 Assuming that a 1 b 2 - a 2 b 1 is not equal to zero, solve the above equation for x to obtain. x = ( c 1 b 2 - c 2 b 1 ) / ( a 1 b 2 - a 2 b 1 ) We can use similar steps to eliminate x and solve for y to obtain. y = ( a 1 c 2 - a 2 c 1 ) / ( a 1 b 2 - a 2 b 1 ) Using the determinant of a Matrix notation, the solution to the given 2 by 2 system of linear equations is given by x = D x / D and y = D y / D where D, D x and D y are the determinants defined by For a 3 by 3 system of linear equations of the form a 1 x + b 1 y + c 1 z = d 1    (1) a 2 x + b 2 y + c 2 z = d 2    (2) a 3 x + b 3 y + c 3 z = d 3    (3) Cramer's rule gives the solution as follows x = D x / D , y = D y / D and z = D z / D where D, D x, D y and D z are determinants defined by Example: Use Cramer's rule for a 3 by 3 system of linear equations to solve the following system 2 x - y + 3 z = - 3 - x - y + 3 z = - 6 x - 2y - z = - 2 Solution: Determinants D, D x, D y and D z are given by (see how to Calculate Determinant of a Matrix.) D = 21 D x = 21 D y = 42 D z = -21 We now use Cramer's rule to find the solution of the system of equations x = D x / D= 1 y = D y / D= 2 z = D z / D= -1 As an exercise check that (1,2,-1) is a solution to the given system of equations. References and links related to systems of equations and determinant.