Question 45·Hard·Linear Equations in One Variable
The equation , where and are constants, has infinitely many solutions.
Which of the following must be true?
I.
II.
III.
For problems about "infinitely many solutions" or identities with linear equations, immediately think: the two sides must be the same linear expression in the variable. Expand and simplify both sides, then either (1) bring all terms to one side and set the coefficient of and the constant term equal to zero, or (2) directly match the -coefficients and constant terms on each side. After finding the necessary parameter values, carefully compare with each roman-numeral statement before picking the answer choice.
Hints
Think about infinitely many solutions for a line
If you have a linear equation in , when will it be true for every value of instead of just one value?
Rewrite the equation in a simpler form
Distribute on the left side to rewrite and then group the terms and constant terms.
Compare both sides as linear expressions
Once both sides are written as something times plus a constant, ask: what conditions on and make these two linear expressions exactly the same for all ?
Use your results to test the statements
After you find the specific values of and that make the equation an identity, see which roman-numeral statements match those values.
Desmos Guide
Graph the difference between both sides
In Desmos, enter the expression y = t*(x - 1) + 3 - (2*x + d) and create sliders for and . This graph shows the difference between the two sides of the equation.
Use the graph to understand infinitely many solutions
Adjust the sliders for and and watch the graph. The equation has infinitely many solutions exactly when the graph of the difference is the horizontal line for all (meaning the two sides are equal for every ). Identify which specific slider values make this happen.
Relate slider values back to the statements
Once you find the values of and that make the graph coincide with , compare those values to statements I, II, and III to see which statements match what you observed in Desmos.
Step-by-step Explanation
Understand what "infinitely many solutions" means
The given equation is
and is the variable. For a linear equation in to have infinitely many solutions, it must be true for every value of . That only happens if the expressions on both sides are identical as functions of (same -coefficient and same constant term).
Expand and simplify both sides in terms of x
First expand the left side:
So the equation becomes
Now you can clearly see the -coefficient and constant term on each side.
Match coefficients for an identity
For the equation
to be true for all :
- The coefficients of must be equal: .
- The constant terms must be equal: .
Substitute into :
So the only way the equation has infinitely many solutions is when and .
Check each statement against the values of t and d
We found that and must both be true for infinitely many solutions.
- Statement I: → true.
- Statement II: → true.
- Statement III: → false, because must equal .
Therefore, the correct choice is I and II only.