6.2
Systems of Linear Equations in Two Variables
Solving a system of linear equations is a fundamental concept in algebra. A system of equations consists of two or more linear equations involving the…
An airplane trip between two countries presents a classic puzzle for solving a system of equations.
The distance is 1260 kilometers. Flying against the wind takes 3 hours, while flying with the wind takes only 2 hours.
Although both trips cover the same distance, the time differs due to the wind’s speed, which remains constant throughout the journey.
Let x be the airplane’s speed in still air, and y be the wind’s speed.
Since distance equals speed multiplied by time, the effective speed against the wind is the airplane’s speed minus the wind’s speed. Multiplying this by 3 hours gives 1260 kilometers.
For the journey with the wind, the effective speed is the airplane’s speed plus the wind’s speed. Multiplying this by 2 hours also gives 1260 kilometers.
These two relationships form a pair of linear equations. Using the elimination method, adding the two equations removes the wind’s speed, leaving a simpler equation in x.
This reveals the airplane’s true speed.
Substituting this value into either equation gives the wind’s speed.
View the full transcript and gain access to JoVE Core videos
Q1: How do you set up a system of linear equations from a real-world problem?
Identify the unknown quantities and assign variables to each. For an airplane trip problem, let x represent the airplane's speed in still air and y represent the wind's speed. Write equations based on the relationship distance equals speed multiplied by time. Against the wind, the effective speed is x minus y; with the wind, it is x plus y. Each scenario produces one equation, forming a system of equations.
Q2: What is the elimination method and how does it simplify solving systems?
The elimination method combines two equations to remove one variable, creating a simpler equation. By adding or subtracting the equations strategically, you cancel out a variable. For example, adding equations where wind speed has opposite signs eliminates that variable, leaving only the airplane's speed to solve for. Once found, substitute this value back into either original equation to find the remaining variable.
Q3: When is the substitution method more practical than elimination?
The substitution method works best when one equation is easily rearranged to isolate a variable. Solve for one variable in terms of the other from one equation, then substitute that expression into the second equation. This approach is particularly useful when coefficients are simple or when one variable already appears isolated, reducing algebraic complexity compared to elimination.
Q4: How do you verify that a solution to a system of equations is correct?
Substitute the calculated values of both variables back into both original equations. If both equations are satisfied—meaning both sides equal—the solution is confirmed correct. For instance, if x equals 27/14 and y equals 19/7, plugging these into each equation should produce true statements, confirming the solution represents the intersection point of the two lines.
Q5: Why does wind speed affect the time required for an airplane journey?
Wind speed changes the airplane's effective speed relative to the ground. Flying against the wind reduces effective speed (airplane speed minus wind speed), requiring more time to cover the same distance. Flying with the wind increases effective speed (airplane speed plus wind speed), reducing travel time. This constant wind speed difference creates two distinct equations that form the system to solve.
Q6: What does the solution to a system of linear equations represent geometrically?
The solution represents the intersection point of the two lines described by the equations on a coordinate plane. Each linear equation graphs as a line, and the x and y values of their intersection satisfy both equations simultaneously. This geometric interpretation confirms that a unique solution exists when two non-parallel lines intersect at exactly one point.
Q7: How do you choose between elimination and substitution methods for solving systems?
Use elimination when coefficients align well for adding or subtracting equations to cancel a variable efficiently. Use substitution when one variable is easily isolated or has a coefficient of one. Both methods yield the same solution; choose based on which requires fewer algebraic steps. For complex systems, gaussian elimination problem solving offers a more systematic approach.