posted by DANIELLE .
Given: in triangle ABC, angle B= 120 degrees
Prove: angle A does not= 60 degrees
Plan: use indirect proof
Assume temporarily that Measure angle A = 60. Then measure angle C=60,
and measure angles A+B+C=180= 60+B+60=180. Therefore the temporary assumption that measure angle A=60 is false. It fallows that measure angle A does not equal 60 degrees.