How do you solve this system of equations using the substitution method #x- y = 1 and 4x + 9y = - 87#?

2 Answers
Nallasivam V
Aug 13, 2018

#y=-7#
#x=-6#

Explanation:

Given -

#x-y=1# ----------------(1)
#4x+9y=-87# ----------(2)
Solve equation (1) for #x#
#x=1+y#
Substitute #x=1+y# in equation (2)
#4(1+y)+9y=-87#
#4+4y+9y=-87#
#13y=-87-4=-91#
#y=(-91)/13=-7#

#y=-7#

Substitute #y=-7# in equation (1)

#x-(-7)=1#
#x+7=1#
#x=1-7=-6#

#x=-6#

maganbhai P.
Aug 13, 2018

#x=-6 and y=-7#

Explanation:

Here ,

#x-y=1 =>x=y+1.....to(1)#

#4x+9y=-87.................to(2)#

Substitute value of #x# from#(1)# into #(2)#

#4(y+1)+9y=-87#

#4y+4+9y=-87#

#13y=-91#

#:.color(red)(y=-7#

Subst. #color(red)(y=-7# into (1)

#x=color(red)(-7)+1=>color(blue)(x=-6#

Hence , #x=-6 and y=-7#

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