document.write( "Question 96755: solve by the substitution method x+3y=-8,4x-3y=23 \n" ); document.write( "
Algebra.Com's Answer #70385 by jim_thompson5910(35256)\"\" \"About 
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Solved by pluggable solver: Solving a linear system of equations by subsitution

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\n" ); document.write( " Lets start with the given system of linear equations
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\n" ); document.write( " \"1%2Ax%2B3%2Ay=-8\"
\n" ); document.write( " \"4%2Ax-3%2Ay=23\"
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\n" ); document.write( " Now in order to solve this system by using substitution, we need to solve (or isolate) one variable. I'm going to choose y.
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\n" ); document.write( " Solve for y for the first equation
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\n" ); document.write( " \"3%2Ay=-8-1%2Ax\"Subtract \"1%2Ax\" from both sides
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\n" ); document.write( " \"y=%28-8-1%2Ax%29%2F3\" Divide both sides by 3.
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\n" ); document.write( " Which breaks down and reduces to
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\n" ); document.write( " \"y=-8%2F3-%281%2F3%29%2Ax\" Now we've fully isolated y
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\n" ); document.write( " Since y equals \"-8%2F3-%281%2F3%29%2Ax\" we can substitute the expression \"-8%2F3-%281%2F3%29%2Ax\" into y of the 2nd equation. This will eliminate y so we can solve for x.
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\n" ); document.write( " \"4%2Ax%2B-3%2Ahighlight%28%28-8%2F3-%281%2F3%29%2Ax%29%29=23\" Replace y with \"-8%2F3-%281%2F3%29%2Ax\". Since this eliminates y, we can now solve for x.
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\n" ); document.write( " \"4%2Ax-3%2A%28-8%2F3%29-3%28-1%2F3%29x=23\" Distribute -3 to \"-8%2F3-%281%2F3%29%2Ax\"
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\n" ); document.write( " \"4%2Ax%2B24%2F3%2B%283%2F3%29%2Ax=23\" Multiply
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\n" ); document.write( " \"4%2Ax%2B8%2B1%2Ax=23\" Reduce any fractions
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\n" ); document.write( " \"4%2Ax%2B1%2Ax=23-8\" Subtract \"8\" from both sides
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\n" ); document.write( " \"4%2Ax%2B1%2Ax=15\" Combine the terms on the right side
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\n" ); document.write( " \"5%2Ax=15\" Now combine the terms on the left side.
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\n" ); document.write( " \"cross%28%281%2F5%29%285%2F1%29%29x=%2815%2F1%29%281%2F5%29\" Multiply both sides by \"1%2F5\". This will cancel out \"5%2F1\" and isolate x
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\n" ); document.write( " So when we multiply \"15%2F1\" and \"1%2F5\" (and simplify) we get
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\n" ); document.write( " \"x=3\" <---------------------------------One answer
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\n" ); document.write( " Now that we know that \"x=3\", lets substitute that in for x to solve for y
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\n" ); document.write( " \"4%283%29-3%2Ay=23\" Plug in \"x=3\" into the 2nd equation
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\n" ); document.write( " \"12-3%2Ay=23\" Multiply
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\n" ); document.write( " \"-3%2Ay=23-12\"Subtract \"12\" from both sides
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\n" ); document.write( " \"-3%2Ay=11\" Combine the terms on the right side
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\n" ); document.write( " \"cross%28%281%2F-3%29%28-3%29%29%2Ay=%2811%2F1%29%281%2F-3%29\" Multiply both sides by \"1%2F-3\". This will cancel out -3 on the left side.
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\n" ); document.write( " \"y=11%2F-3\" Multiply the terms on the right side
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\n" ); document.write( " \"y=-11%2F3\" Reduce
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\n" ); document.write( " So this is the other answer
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\n" ); document.write( " \"y=-11%2F3\"<---------------------------------Other answer
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\n" ); document.write( " So our solution is
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\n" ); document.write( " \"x=3\" and \"y=-11%2F3\"
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\n" ); document.write( " which can also look like
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\n" ); document.write( " (\"3\",\"-11%2F3\")
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\n" ); document.write( " Notice if we graph the equations (if you need help with graphing, check out this solver)
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\n" ); document.write( " \"1%2Ax%2B3%2Ay=-8\"
\n" ); document.write( " \"4%2Ax-3%2Ay=23\"
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\n" ); document.write( " we get
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\n" ); document.write( " graph of \"1%2Ax%2B3%2Ay=-8\" (red) and \"4%2Ax-3%2Ay=23\" (green) (hint: you may have to solve for y to graph these) intersecting at the blue circle.
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\n" ); document.write( " and we can see that the two equations intersect at (\"3\",\"-11%2F3\"). This verifies our answer.
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\n" ); document.write( " Check:
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\n" ); document.write( " Plug in (\"3\",\"-11%2F3\") into the system of equations
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\n" ); document.write( " Let \"x=3\" and \"y=-11%2F3\". Now plug those values into the equation \"1%2Ax%2B3%2Ay=-8\"
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\n" ); document.write( " \"1%2A%283%29%2B3%2A%28-11%2F3%29=-8\" Plug in \"x=3\" and \"y=-11%2F3\"
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\n" ); document.write( " \"3-33%2F3=-8\" Multiply
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\n" ); document.write( " \"-24%2F3=-8\" Add
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\n" ); document.write( " \"-8=-8\" Reduce. Since this equation is true the solution works.
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\n" ); document.write( " So the solution (\"3\",\"-11%2F3\") satisfies \"1%2Ax%2B3%2Ay=-8\"
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\n" ); document.write( " Let \"x=3\" and \"y=-11%2F3\". Now plug those values into the equation \"4%2Ax-3%2Ay=23\"
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\n" ); document.write( " \"4%2A%283%29-3%2A%28-11%2F3%29=23\" Plug in \"x=3\" and \"y=-11%2F3\"
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\n" ); document.write( " \"12%2B33%2F3=23\" Multiply
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\n" ); document.write( " \"69%2F3=23\" Add
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\n" ); document.write( " \"23=23\" Reduce. Since this equation is true the solution works.
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\n" ); document.write( " So the solution (\"3\",\"-11%2F3\") satisfies \"4%2Ax-3%2Ay=23\"
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\n" ); document.write( " Since the solution (\"3\",\"-11%2F3\") satisfies the system of equations
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\n" ); document.write( " \"1%2Ax%2B3%2Ay=-8\"
\n" ); document.write( " \"4%2Ax-3%2Ay=23\"
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\n" ); document.write( " this verifies our answer.
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