How do you solve by substitution y=3xy=3x and x+y=-5x+y=−5?

2 Answers
Ridinion K.
May 6, 2018

x=-1.25x=−1.25 and y=-3.75y=−3.75

Explanation:

Using y=3xy=3x substitute this value of yy into x+y=-5x+y=−5:

x+y=-5x+y=−5

x+3x=-5x+3x=−5

Then simplify and rearrange to make xx the subject of the equation:

x+3x=-5x+3x=−5

4x=-54x=−5

x=-5/4x=−54

x=-1.25x=−1.25

Then substitute this value of xx back into y=3xy=3x so you find the value of yy (if it asks you for it):

y=3(-1.25)y=3(−1.25)

y= -3.75y=−3.75 or in fractions y=-15/4y=−154

To double check your answer, substitute values of xx and yy into x+y=-5x+y=−5 to see whether you get the result of -5−5:

x+y=-5x+y=−5

-1.25+(-3.75)=-5−1.25+(−3.75)=−5

KB.
May 6, 2018

x=-5/4x=−54
y=-15/4y=−154

Explanation:

y=3xy=3x
x+y=-5x+y=−5

As we see yy is equal to 3x3x in the first equation.
yy which is in the second equation must be replaced or substituted by 3x.3x.

So
y=color(red)(3x)y=3x
x+color(red)y=-5x+y=−5

x+color(red)(3x)=5x+3x=5
4x=-54x=−5
x=-5/4x=−54

y=3x=3*(-5/4)=-15/4y=3x=3⋅(−54)=−154

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