Inconsistent systems example

Example Solve the given systems of equations.

x - y = 6

-2x + 2 y = 1

Solution

We can utilize either method here, although it looks like substitution would possibly be slightly easier.

We'll solve out the first equation for x & substitute that in the second equation.

x = 6 + y

-2 (6 + y )+ 2 y = 1

-12 - 2 y + 2 y = 1

-12 =1  ??

Thus, this is clearly not true and there doesn't seem to be a mistake anywhere in our work.  Hence, what's the problem?  To see let's graph these two lines and illustrates what we get.

It seem that these two lines are parallel (can you check that with the slopes?) and we know that two parallel lines along with different y-intercepts (that's significant) will never cross.

Since we saw in the opening discussion of this section solutions revel the point where two lines intersect.  If two lines don't intersect we can't comprise a solution.

Thus, when we get this kind of nonsensical answer from our work we contain two parallel lines and there is no solution to this system of equations.

This system is called inconsistent.  Note that if we'd utilized elimination on this system we would have ended up with a similar nonsensical answer.