Question ID: f75bd744
Assessment Test Domain Skill Difficulty
SAT Math Algebra Systems of two linear
equations in two
variables
Hard
Question
In the given system of equations, is a constant. If the system has no solution, what is the value of ?
Correct Answer: 8
Rationale
The correct answer is . The given system of equations can be solved using the elimination method. Multiplying both sides of the second
equation in the given system by yields , or . Adding this equation to the first equation in the given
system, , yields , or . Subtracting from
both sides of this equation yields , or . If the given system has no
solution, then the equation has no solution. If this equation has no solution, the coefficients of on each side of the
equation,
îš•
and , must be equal, which yields the equation . Dividing both sides of this equation by yields . Thus, if
the system has no solution, the value of is .
Alternate approach: A system of two linear equations in two variables, and , has no solution if the lines represented by the equations in the xy-
plane are parallel and distinct. Lines represented by equations in the form , where , , and are constant terms, are parallel if
the ratio of the x-coefficients is equal to the ratio of the y-coefficients, and distinct if the ratio of thex
-coefficients are not equal to the ratio of the
constant terms. Subtracting from both sides of the first equation in the given system yields , or
. Subtracting from both sides of the second equation in the given system yields , or
. The ratio of the x-coefficients for these equations is , or . The ratio of the y-coefficients for these equations is . The ratio
of the constant terms for these equations is
, or . Since the ratio of the x-coefficients, , is not equal to the ratio of the constants, , the
lines represented by the equations are distinct. Setting the ratio of the
x
-coefficients equal to the ratio of the y-coefficients yields .
Multiplying both sides of this equation by
îš•
yields , or . Therefore, when
îš•
, the lines represented by
these equations are parallel. Thus, if the system has no solution, the value of is .