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Solving Pairs of Equations Help

By — McGraw-Hill Professional
Updated on Oct 3, 2011

Introduction to Solving Pairs of Equations

The solutions of pairs of equations can be envisioned and approximated by graphing both of the equations on the same set of coordinates. Solutions appear as intersection points between the plots.

A Line And A Curve

Suppose you are given two equations in two variables, such as x and y , and are told to solve for values of x and y that satisfy both equations. Such equations are called simultaneous equations . Here is an example:

y = x 2 + 2 x + 1

y = − x + 1

These equations are graphed in Fig. 6-12. The graph of the first equation is a parabola, and the graph of the second equation is a straight line. The line crosses the parabola at two points, indicating that there are two solutions of this set of simultaneous equations. The coordinates of the points, corresponding to the solutions, can be estimated from the graph. It appears that they are approximately:

( x 1 , y 1 ) = (−3, 4)

( x 2 , y 2 ) = (0, 4)

You can solve the pair of equations using plain algebra, and determine the solutions exactly.

The Cartesian Plane Solving Pairs of Equations A Line And A Curve

Fig. 6-12 . Graphs of two equations, showing solutions as intersection points.

Another Line And Curve

Here is another pair of “two-by-two” equations (two equations in two variables) that can be approximately solved by graphing:

y = −2 x 2 + 4 x − 5

y = −2 x − 5

These equations are graphed in Fig. 6-13. Again, the graph of the first equation is a parabola, and the graph of the second equation is a straight line. The line crosses the parabola at two points, indicating that there are two solutions. The coordinates of the points, corresponding to the solutions, appear to be approximately:

( x 1 , y 1 ) = (3, −11)

( x 2 , y 2 ) = (0, −5)

Again, if you want, you can go ahead and solve these equations using algebra, and find the values exactly. 

The Cartesian Plane Solving Pairs of Equations Another Line And Curve

Fig. 6-13 . Another example of equation solutions shown as the intersection points of their graphs.

How Many Solutions?

Graphing simultaneous equations can reveal general things about them, but should not be relied upon to provide exact solutions. In real-life scientific applications, graphs rarely show exact solutions unless they are so labeled and represent theoretical ideals.

A graph can reveal that a pair of equations has two or more solutions, or only one solution, or no solutions at all. Solutions to pairs of equations always show up as intersection points on their graphs. If there are n intersection points between the curves representing two equations, then there are n solutions to the pair of simultaneous equations.

If a pair of equations is complicated, or if the graphs are the results of experiments, you’ll occasionally run into situations where you can’t use algebra to solve them. Then graphs, with the aid of computer programs to closely approximate the points of intersection between graphs, offer a good means of solving simultaneous equations.

Sometimes you’ll want to see if a set of more than two equations in x and y has any solutions shared by them all. It is common for one or more pairs of a large set of equations to have some solutions; this is shown by points where any two of their graphs intersect. But it’s unusual for a set of three or more equations in x and y to have any solutions when considered all together (that is, simultaneously). For that to be the case, there must be at least one point that all of the graphs have in common.

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