Question bank / Linear inequalities in one or two variables

Math · AlgebraHard

y<−2x+8y < -2x + 8
y>x+cy > x + c
In the system of inequalities, cc is a constant. If the point (2,3)(2, 3) is a solution to the system, which of the following must be true?

A

c<−1c < -1

B

c>−1c > -1

C

c<1c < 1

D

c>1c > 1

Show answer and explanation

Answer: C

The first inequality gives 3<−2(2)+8=43 < -2(2) + 8 = 4, which is true no matter what cc is. The second inequality requires 3>2+c3 > 2 + c, so c<1c < 1. Values such as c=0c = 0 and c=−5c = -5 both make (2,3)(2, 3) a solution, so neither choice A nor choice B has to be true.

Choice D reverses the inequality when solving 3>2+c3 > 2 + c.

Why the other choices are wrong

  • A. c could be 0, which still makes (2, 3) a solution, so c < -1 doesn't have to be true.
  • B. c could be -5, which also makes (2, 3) a solution, so c > -1 doesn't have to be true.
  • D. This reverses the inequality. From 3 > 2 + c, subtracting 2 gives 1 > c.
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