SOLUTION: use cramer's rule to solve the linear system. show the set up. x -2y=1 2x +y=7

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Question 253660: use cramer's rule to solve the linear system. show the set up.
x -2y=1
2x +y=7
Found 2 solutions by jim_thompson5910, Edwin McCravy:
Answer by jim_thompson5910(35256) About Me  (Show Source):
You can put this solution on YOUR website!
Solved by pluggable solver: Using Cramer's Rule to Solve Systems with 2 variables



system%281%2Ax%2B-2%2Ay=1%2C2%2Ax%2B1%2Ay=7%29



First let A=%28matrix%282%2C2%2C1%2C-2%2C2%2C1%29%29. This is the matrix formed by the coefficients of the given system of equations.


Take note that the right hand values of the system are 1 and 7 which are highlighted here:
system%281%2Ax%2B-2%2Ay=highlight%281%29%2C2%2Ax%2B1%2Ay=highlight%287%29%29



These values are important as they will be used to replace the columns of the matrix A.




Now let's calculate the the determinant of the matrix A to get abs%28A%29=%281%29%281%29-%28-2%29%282%29=5. Remember that the determinant of the 2x2 matrix A=%28matrix%282%2C2%2Ca%2Cb%2Cc%2Cd%29%29 is abs%28A%29=ad-bc. If you need help with calculating the determinant of any two by two matrices, then check out this solver.



Notation note: abs%28A%29 denotes the determinant of the matrix A.



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Now replace the first column of A (that corresponds to the variable 'x') with the values that form the right hand side of the system of equations. We will denote this new matrix A%5Bx%5D (since we're replacing the 'x' column so to speak).


A%5Bx%5D=%28matrix%282%2C2%2Chighlight%281%29%2C-2%2Chighlight%287%29%2C1%29%29



Now compute the determinant of A%5Bx%5D to get abs%28A%5Bx%5D%29=%281%29%281%29-%28-2%29%287%29=15. Once again, remember that the determinant of the 2x2 matrix A=%28matrix%282%2C2%2Ca%2Cb%2Cc%2Cd%29%29 is abs%28A%29=ad-bc



To find the first solution, simply divide the determinant of A%5Bx%5D by the determinant of A to get: x=%28abs%28A%5Bx%5D%29%29%2F%28abs%28A%29%29=%2815%29%2F%285%29=3



So the first solution is x=3




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We'll follow the same basic idea to find the other solution. Let's reset by letting A=%28matrix%282%2C2%2C1%2C-2%2C2%2C1%29%29 again (this is the coefficient matrix).




Now replace the second column of A (that corresponds to the variable 'y') with the values that form the right hand side of the system of equations. We will denote this new matrix A%5By%5D (since we're replacing the 'y' column in a way).


A%5Bx%5D=%28matrix%282%2C2%2C1%2Chighlight%281%29%2C2%2Chighlight%287%29%29%29



Now compute the determinant of A%5By%5D to get abs%28A%5By%5D%29=%281%29%287%29-%281%29%282%29=5.



To find the second solution, divide the determinant of A%5By%5D by the determinant of A to get: y=%28abs%28A%5By%5D%29%29%2F%28abs%28A%29%29=%285%29%2F%285%29=1



So the second solution is y=1




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Final Answer:




So the solutions are x=3 and y=1 giving the ordered pair (3, 1)




Once again, Cramer's Rule is dependent on determinants. Take a look at this 2x2 Determinant Solver if you need more practice with determinants.

Answer by Edwin McCravy(20054) About Me  (Show Source):
You can put this solution on YOUR website!

Learn the Cramer's rule setup:

system%28Ax+%2B+By+=+C%2C%0D%0A++++++++++Dx+%2B+Ey+=+F%29





system%28x-2y=1%2C2x%2By=7%29

Put in the understood 1 coefficients

system%281x-2y=1%2C2x%2B1y=7%29

A=1, B=-2, C=1, D=2, E=1, F=7





Edwin