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

2 Answers
May 6, 2018

x=-1.25 and y=-3.75

Explanation:

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

x+y=-5

x+3x=-5

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

x+3x=-5

4x=-5

x=-5/4

x=-1.25

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

y=3(-1.25)

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

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

x+y=-5

-1.25+(-3.75)=-5

May 6, 2018

x=-5/4
y=-15/4

Explanation:

y=3x
x+y=-5

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

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

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

y=3x=3*(-5/4)=-15/4