Which pair of triangles must be similar?
A. Triangles 1 and 2 each have a 35° angle.
B. Triangles 3 and 4 are both isosceles. They each have a 40° angle.
C. Triangle 5 has a 30° angle and a 90° angle. Triangle 6 has a 30° angle and a
70° angle.
D. Triangle 7 has a 50° angle and a 25° angle. Triangle 8 has a 50° angle and a
105° angle.

Correct  B. and D.

Explanation:

This question is from mathematics and not from physics.

According to the triangle similarity theorem the two triangles are similar if the two angles of one triangle are congruent with the two angles of the other triangle.

B.

Isosceles triangles as a special case are similar if they have one angle the same and that is an angle of 40°.

D.

Triangle 7 has angles 50°, 25° and 105°

Triangle 8 has angles 50°, 105° and 25°

So they have all three angles congruent and therefore they are similar.

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