Triangles

Look at triangle ABC. What do angles 1, 2, and 3 form? What is the sum of their measure?

The line through points A and B and the line through point C are parallel. The line through A and C and the line through B and C are transversals. What do we know about the acute angles formed by a transversal that intersects two parallel lines? The obtuse angles? From this what can we say about the measures of angles 1 and 2 in relation to angles A and B? Knowing the measures of angles 1 and 2, what is the measure of 3? What is the sum of the measures of the angles of a triangle?

The sum of the measures of a triangle is 180°. Drag points A, B, and C around an convince yourself the sum of the angles is always 180°.

Determine the measure of the missing angle in each of the triangles.

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Check your answers by clicking in the boxes next to the triangles. How did you do?

Michael Smith, Created with GeoGebra