Does dx in definite integrals have to be 1.

Given: y║z

Using given statement:

From the figure, we have

By the definition of straight angle, .

By the angle addition postulates in we have;

The Alternate Interior Angles theorem states:

If two parallel lines are cut by a transversal, then the pairs of alternate interior angles are congruent.

Since , then by alternative interior angle theorem we have  

and

We know that : then by the substitution of and we get,