How do you solve #4x-3y = -15# and #x + y = 5 # using substitution?

1 Answer
EET-AP · EZ as pi
Feb 11, 2017

#x = 0; " "y = 5#

Explanation:

Given:
#4x -3y = -15color(white)(..................)A#
# x +y = 5color(white)(............................)B#

Then, from equation B:
#color(blue)(x = 5 - y)#

Substitute for #x# in equation A;
#4color(blue)((5 - y)) -3y = -15#
#20 - 4y -3y = -15#
#20 -7y = -15#
#-7y = -15 -20" "# multiply both sides by #-1#
# 7y = 35#
#y = 5#

Substitute the value of #y# in equation B:
#x +y = 5#
#x +5 =5#
#x = 5 - 5#
#x = 0#

To check, substitute the values into equation A":
#4x -3y = -15#
#4(0) -3(5) = - 15#
#-15 = -15#

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