Adrian is using the indirect method to prove that segment DE is not parallel to segment BC in the triangle ABC shown below:

If we start with the assumption that segment DE is parallel to segment BC, then we the similarity condition that:

triangle ABC is similar to triangle ADE

For this to be true, the ratio corresponding sides of the two triangles must be equal. So:

5/8 =? 6/10

But

5/8 =/= 6/10

So, DE is not parallel to BC

Hope this helps you

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