Triangle abc is an obtuse triangle with the obtuse angle at vertex b. angle a must be

less than 90°.
greater than 90°.
congruent to angle b.
congruent to angle c.

Answer would be first choice, less than 90°.

The angles within a triangle add up to 180°.  If an angle is obtuse in the triangle (greater than 90°), it is impossible for the other angles to be obtuse or equal to 90°.

Hope this helps!

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