Solve the simultaneous equations in real numbers: 6x + 3y = 7xy and 3x + 9y = 11xy. Find the ordered pair (x, y) that satisfies both equations (choose the non-trivial solution if more than one arises).

Difficulty: Medium

Correct Answer: x = 1, y = 3/2

Explanation:


Introduction:
We are given two equations with product terms (xy), which suggests rearranging to isolate either x or y, or converting to a form that allows elimination. The task is to find the ordered pair (x, y) that satisfies both equations simultaneously. Non-trivial solutions are preferred if multiple solutions arise.


Given Data / Assumptions:

  • Equations: 6x + 3y = 7xy and 3x + 9y = 11xy.
  • Real-number solutions; trivial (x, y) = (0, 0) may appear but the meaningful solution has nonzero variables.
  • Standard algebraic operations are valid.


Concept / Approach:
Treat each equation as linear in one variable after moving the xy-term to one side. Factor where possible, then eliminate using substitution or simultaneous methods. Because xy multiplies both variables, dividing by a variable is permitted only if that variable is nonzero; we keep track of this carefully.


Step-by-Step Solution:

Equation (1): 6x + 3y = 7xy → bring terms together.Equation (2): 3x + 9y = 11xy → bring terms together.From (1): 7xy − 6x − 3y = 0 → factor by grouping is difficult; switch to elimination.From (2): 11xy − 3x − 9y = 0.Treat (1) as linear in x: 7xy − 6x = 3y → x(7y − 6) = 3y → x = (3y)/(7y − 6), provided 7y − 6 ≠ 0.Substitute into (2): 3x + 9y = 11xy.3*(3y/(7y − 6)) + 9y = 11*(3y/(7y − 6))*y.Multiply throughout by (7y − 6) to clear denominators and solve; this yields y = 3/2 (nonzero) and then x = 1.A trivial solution (0, 0) also satisfies both, but the non-trivial solution is (1, 3/2).


Verification / Alternative check:
Substitute x = 1, y = 3/2 into both equations: (1) 6*1 + 3*(3/2) = 6 + 4.5 = 10.5; 7*1*(3/2) = 10.5. (2) 3*1 + 9*(3/2) = 3 + 13.5 = 16.5; 11*1*(3/2) = 16.5. Both hold exactly.


Why Other Options Are Wrong:
Each alternative fails one (or both) equations upon substitution; signs and magnitudes do not satisfy both identities simultaneously.


Common Pitfalls:
Dividing by zero inadvertently or discarding valid solutions. Always verify candidates in both equations.


Final Answer:
x = 1, y = 3/2

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