Question

Find the angle between x+y=0 and x-y=0?

Answer 1

`90` degrees.

Explanation:

Let's write each function in the slope-intercept form of `y=mx+b`

`x+y=0`

=>`y=-x`

The slope is `-1`

`x-y=0`

=>`-y=-x`

=>`y=x`

The slope is `1`

Now, remember that if the slope between the two lines is the negative reciprocal of each other, then the lines are perpendicular.

Also, remember that `b` is a negative reciprocal of `a` when `b=-1/a`.

We ask ourselves: is `1` a negative reciprocal of `-1`?

Let's see...

`-1/1=-1=>-1=-1` Aha! the lines must be perpendicular!

Therefore, the angle between the two lines is `90` degrees.