Answer:
Because Alternate Interior angles are equal.
Explanation:
STEP - I:

Reason:
This is the given data.
STEP - II:
It has multiplied and the reason is mentioned as well.
STEP - III:
= 45° + 135° = 180°
Reason:
It has substituted the value of
from the previous step. The sum of
and
is 180°.
STEP - IV:

Reason:
We calculated
to be 135°.
Note that
and
are on the same line. That means their sum should be 180°.
i.e.,
= 180°.
135° +
= 180°.
45°.
One of the ways to prove
is to check if alternate interior angles are equal.
Here,
and
are alternate interior angles and they are equal.
.