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Use addition or substitution to find the value of y for this set of equations. top line 4x + 13y = 40 bottom line 4x + 3y = -40

Use addition or substitution to find the value of y for this set of equations. top line 4x + 13y = 40 bottom line 4x + 3y = -40

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George P. | Penn State Instructor exp.: Algebra, Trig., PreCalculus, and CalculusPenn State Instructor exp.: Algebra, Tri...
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Using the addition method, you could first multiply one equation by -1

then add to eliminate the x variables. 

Let's multiply the second equation by -1 to give us:

4x   +  13y  =  40

-4x  +  (-3y) =  40  

adding gives us

           10y  =  80

               y  =  8  (dividing both sides by 8)

If you also needed to find x, you could then substitute

the value of 8 in for y in one of the equations and solve for x.

 

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