The triangle ABC, with angle measures of 40°, 40°, and 100°, can be classified based on its angles and sides.
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Angle Classification: Since one of the angles (100°) is greater than 90°, the triangle is classified as an obtuse triangle.
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Side Classification: Given that the triangle has exactly two congruent sides (the angles of 40° indicate the sides opposite them are equal), it is also classified as an isosceles triangle.
Therefore, triangle ABC is classified as an isosceles obtuse triangle.