Question 226·Medium·Nonlinear Equations in One Variable; Systems in Two Variables
The equation relates the positive numbers and . Which equation correctly expresses in terms of ?
For proportion-style equations (one fraction equals another), the fastest approach on the SAT is to clear denominators by cross-multiplying, then treat the result as a regular linear equation. Expand carefully, collect all terms on one side and constants on the other, factor out if needed, and divide by its coefficient. Always watch signs when moving terms across the equals sign, and if answer choices are given in fractional form, compare your final expression directly to those choices before bubbling in.
Hints
Use the structure of a proportion
You have one fraction equal to another: . How can you eliminate the denominators in one step?
Work with the equation without fractions
After you clear the fractions, you will get an equation with parentheses on both sides. Expand each side and then collect all the terms on one side.
Factor out x at the end
Once you have an equation of the form , factor out if needed and then divide to solve for .
Desmos Guide
Pick a simple value for k
Choose an easy positive value for that does not make any denominator zero (so avoid and ). For example, type k = 3 in Desmos to fix at 3.
Compute x from each answer choice
In Desmos, define four separate expressions using your chosen , such as xA = (2k+3)/(6-k), xB = (2k-3)/(6-k), xC = (6-k)/(2k+3), and xD = (2k+3)/(k-6). Desmos will show numerical values for each of these.
Test which x satisfies the original equation
For each candidate (for example, xA), type 3/k and (xA+2)/(2*xA-1) as two separate expressions and compare their numerical values. Repeat for xB, xC, and xD. The choice whose value makes and equal is the correct expression for in terms of .
Step-by-step Explanation
Clear the fractions by cross-multiplying
The equation is
Because this is a proportion (one fraction equals another), multiply both sides by to eliminate denominators:
Expand and group like terms
Now expand both sides:
So the equation becomes
Get all the terms on one side and constants on the other. Subtract from both sides and add to both sides:
Factor on the left:
Solve for x and match the answer choice
To isolate , divide both sides by (note so the denominator is not zero):
This expression matches choice A: . This is the correct way to express in terms of .