Visually verifying a solution to a system | Grade 8 (TX TEKS) | Khan Academy

- [Instructor] We're told the
system of linear equations

below is graphed on the coordinate grid.

Okay, we see that.

Then, it says: What is the value

of y equals -x - 7

when x is equal to -8.

So the graph y equals -x - 7,

that's this blue line here,

and they tell us, right?

That the point -8, 1 is on that line.

So when x is equal to -8,

y is equal to 1.

So we could type that in.

And then, they ask us:

What is the y-value of the bronze line

or the brown line here,

y is equal to 1/2x - 1

when x is equal to -8.

So we don't even have to compute it.

We see when x is equal to -8,

we get y is equal to -5,

so we put -5 there.

And then, they say:

Is x equal -8 a solution?

Well, the only way that x
equals -8 would be a solution

is if the y value of those lines

when x equals -8 were the same,

but they're clearly not.

And in fact, we would see it visually.

If it was a solution,

the two lines would intersect
at that point but they don't.

There's actually two
different points here.

They don't overlap.

So this is not a solution

and we're done.