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How do you solve by substitution $x-y+-15$ and $x+y=-5$ ?

1 Answer

Jun 5, 2015

Assuming the first component should have ( $=-$ ) instead of ( $+-$ )

[1] $color(white)("XXXX")$ $x-y = -15$
[2] $color(white)("XXXX")$ $x+y=-5$

Rearranging the terms of [1]
[3] $color(white)("XXXX")$ $x= y-15$

Substituting $(y-15)$ for $x$ into [2]
[4] $color(white)("XXXX")$ $(y-15) + y = -5$
[5] $color(white)("XXXX")$ $y=5$

Substituting $5$ for $y$ back in [1]
[6] $color(white)("XXXX")$ $x - 5 = -15$
[7] $color(white)("XXXX")$ $x = -10$

Solution: $(x,y) = (-10,5)$

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