Question 42·Hard·Linear Equations in One Variable
The equation below involves the constant .
For what value of does the equation have no solution?
(Express the answer as an integer)
For SAT questions asking when a linear equation in one variable has no solution or infinitely many solutions, first fully simplify the equation so all x-terms and constants are combined. Rewrite it in the form , where and depend on the parameter (here, ). Then use the key facts: if there is one solution; if and there are infinitely many solutions; if and there is no solution. Set the coefficient of equal to zero, check what happens to the constant term, and solve for the parameter accordingly. This avoids messy plugging and gives the answer quickly and reliably.
Hints
Start by simplifying the equation
Distribute over on the left-hand side so that both sides of the equation are expanded.
Get all x-terms together
After expanding, move all terms to one side and all constant (no ) terms to the other side so the equation looks like "(something) = (something)."
Think about when a linear equation has no solution
If you have an equation of the form in the variable , what must be true about and for there to be no possible value of that makes the equation true?
Apply that idea to this problem
In your simplified equation, identify the coefficient of and the constant. Which value of makes the coefficient of zero while the constant term is not zero?
Desmos Guide
Graph both sides as lines with a k-slider
In one expression line, type y1 = (2k - 1)(x - 3) + 7. In another line, type y2 = (k + 5)x + 5k - 6. Desmos will create a slider for k.
Use the slider to see when there is no intersection
Move the k slider and watch the graphs of y1 and y2. You are looking for the value of k where the two lines are parallel (same slope) but never intersect. Read that k value from the slider.
Confirm with a table (optional)
For that k value, create a table for y1 and y2 at several x-values. If the y-values are never equal for any x, that k value makes the original equation have no solution.
Step-by-step Explanation
Expand both sides of the equation
Start by distributing on the left side.
Left side:
Right side is already expanded:
So the equation becomes
Collect like terms to get a single linear equation in x
Move all terms to one side so the equation is in the form (something) + (something) .
Subtract and from both sides:
Combine like terms:
- Coefficient of :
- Constant term:
So the equation becomes
Rewriting, this is equivalent to
Use the condition for a linear equation to have no solution
Think of the simplified equation as , where and depend on .
Here,
- (the coefficient of )
- (the constant term)
For an equation in the variable :
- If , there is exactly one solution.
- If and , there are infinitely many solutions.
- If and , there is no solution.
So for no solution, we need
- (so ), and
- (so ).
Now find the value of that makes , and then check that is not zero for that value.
Solve for k and check the condition
Solve :
Check at :
so the condition and is satisfied.
To confirm, substitute back into the original equation:
Left side:
Right side:
The equation becomes
which simplifies to , an impossible statement. That means there is no solution in when .
So the value of that makes the equation have no solution is .