Posted by aneme on Tuesday, February 12, 2008 at 9:42pm.
Is the second line supposed to be 7x + y = -8, or 7x - y = -8? You sweem to have inserted a second equal sign by mistake.
Whichever it is, use the second equation to derive an equation for y in terms of x. Then substitute that "y" value into the first equation, and it will be an equation in x only. Solve it.
7x+y=-8
i don't understand what you mean. could you possibly do an example?
We have this:
How do you use the substitution method to solve the linear system?
-2x - 5y = 7...Equation A
7x - y = -8...Equation B
The idea is to isolate y OR x (your choice) and then plug that into EITHER Equation A or B (your choice).
I will isolate y in Equation B.
7x - y = -8...Equation B
-y = -7x - 8
y = (-7x - 8)/-1
y = 7x + 8
Do you see that we NOW know that y is 7x + 8?
To find x, I will plug 7x + 8 into EITHER Equation A or B.
I will choose Equation A.
-2x-5y = 7...Equation A
-2x - 5(2x + 8) = 7
-2x - 10x - 40 = 7
-12x - 40 = 7
-12x = 7 + 40
-12x = 47
x = 47/-12
x = -47/12...Value for x.
We just found the value of x and it is
-47/12. See it above?
We now plug our value for x into EITHER Equation A or B to find the value of y.
I will choose Equation A again.
-2x -5y = 7...Equation A
-2(-47/12) - 5y = 7
47/6 - 5y = 7
-5y = -47/6 + 7
-5y = -5/6
y = -5/6 divided by -5
y = 1/6
The solution for this system of linear equations in two variables is
x = -47/12 and y = 1/6.
We can also write the solution this way:
(-47/12, 1/6) = (x, y), where x represents -47/12 and y represents 1/6.
Done!
whats the answer to -2x-5y=7
7x+y=-8