Question: Parallel or Not

Comment on Parallel or Not

Can you please explain how when Golden angles and Purple angles together if it is 180 degree, the two lines are parallel ?
gmat-admin's picture

The important thing to recognize that the rule for parallel lines goes in BOTH directions. Here's what I mean.

IF the two lines are parallel, THEN we know that ∠e + ∠b = 180 degrees.

We can also say that, IF ∠e + ∠b = 180 degrees, THEN the two lines are parallel.

More here: https://www.gmatprepnow.com/module/gmat-geometry/video/858

Can you explain this one , As ∠e can be 60 and ∠b can 120 that makes to total of 180 . but its still then the lines are not parallel .
gmat-admin's picture

If ∠e = 60 and ∠b = 120, then the two lines are definitely parallel.

Notice that, if ∠e = 60, then ∠d = 120 (since angles on a line add to 180 degrees)

This means ∠d = ∠b

If ∠d = ∠b, then we can be certain that lines 1 and 2 are parallel.

Conversely, we can also say that, if lines 1 and 2 are parallel, then it must be the case that ∠d = ∠b

For more on this, watch https://www.gmatprepnow.com/module/gmat-geometry/video/858 (starting at 5:40)

Does that help?

Cheers,
Brent

Okay make sense , thanks for the help

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