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

2 Answers
May 6, 2018

Answer:

#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

Answer:

#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#