Answer:
13
Step-by-step explanation:
Perhaps the most straightforward way to find the value of the given function is to first solve the equations for x and y. The results can be substituted into your expression to find the value you want.
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We notice the y-coefficients are opposites, so we can eliminate the y-variable by adding the equations.
(2x -y) +(x +y) = (3) +(1)
3x = 4 . . . . . . simplify
x = 4/3 . . . . . divide by 3
The value of y can be found from the second equation:
4/3 +y = 1 . . . . substitute for x
y = -1/3 . . . . . . subtract 4/3
The value of 8x-7y is found by substituting the x- and y-values into this expression:
8(4/3) -7(-1/3) = 32/3 +7/3 = 39/3 = 13
The value of 8x-7y is 13.
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Additional comment
You can also work out what combination of the two equations will give 8x -7y. That turns out to be 5 times the first minus 2 times the second:
5(2x -y) -2(x +y) = 5(3) -2(1)
10x -5y -2x -2y = 15 -2
8x -7y = 13
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The coefficients 5 and -2 are found by solving the system of equations ...
a(2x -y) +b(x +y) ≡ 8x -7y ⇒ {2a +b = 8, -a +b = -7}
