Angles in a Triangle

I was team teaching with a trainee today and she asked a question similar to the bottom middle question. The students found the unknown angle to be 70° and she asked ‘how did you know you could halve 140° to get the unknown angle?’ Students gave the answer ‘you take 70° from 180° and then you always halve whats left’ The trainee questioned that ‘do you always halve? What was it you saw in this question that let you know you could do this?’

So It made me think about questions that could have answer of 70° sometimes, always or never.

Screen Shot 2019-03-04 at 21.10.17