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Determine if the biconditional is true. If false, give a counterexample.
An angle is a right angle if and only if it is not obtuse.

false; m∠1 = 120°, ∠1 is not a right angle

false; m∠1 = 70°, ∠1 is not a right angle

false; m∠1 = 90°, ∠1 is not an obtuse angle

true

Answer :

"An angle is a right angle if and only if it is not obtuse"
can be broken down into
"if an angle is a right angle, then it is not obtuse" AND "if an angle is not obtuse, then it is a right angle"

Let's focus on the second statement
"if an angle is not obtuse, then it is a right angle"

This is false. One such example is if the angle is 70 degrees as choice B is showing. This is not a 90 degree angle so it's not a right angle, but it's also not obtuse. 

Final Answer: Choice B) 

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