Question 30·200 Super-Hard SAT Math Questions·Algebra
Consider the equation
If is a constant and the equation has at least one solution, how many solutions does the equation have?
When a linear equation contains a parameter like , expand and simplify first, then compare coefficients. If the -terms cancel, you’re not solving for anymore—you’re checking whether the remaining constants match. A true statement (like ) means infinitely many solutions, and a false statement (like ) means no solutions. If the problem tells you there is at least one solution, that rules out the “false statement” case immediately.
Hints
Expand carefully
Distribute across both terms inside .
Look for cancellation
After expanding, compare the -term on the left with the -term on the right. See what happens when you subtract the same -term from both sides.
Use the phrase “has at least one solution”
If the -terms cancel, the equation becomes a statement about only. A solution exists only when that statement is true.
Desmos Guide
Enter each side as a function
In Desmos, enter
When you type , create it as a slider.
Test values of
Move the slider for and watch the graphs.
- For many values, the two lines will be parallel and never intersect (no solutions).
- Adjust until the two graphs lie exactly on top of each other (same line).
Connect the graph behavior to number of solutions
When the two graphs coincide, every works, so the equation has infinitely many solutions. Use this observation to select the correct choice.
Step-by-step Explanation
Distribute on the left side
Expand the left side:
Write the equation and cancel matching -terms
Substitute the expanded left side into the equation:
Subtract from both sides. The -terms cancel, leaving a condition on :
Find the value of that allows at least one solution
Solve the remaining equation:
So the equation has at least one solution only when . Otherwise, it would say a false statement like , which has no solutions.
Determine how many solutions occur when
When , both sides become the same expression in :
That means every real value of satisfies the equation, so the equation has infinitely many solutions.