Respostas

2014-04-29T10:23:02-03:00
 \alpha + \beta =180
 \alpha - \beta =60
 \alpha =180- \beta
 180-\beta - \beta =60
 -\beta - \beta =60-180
-2 \beta =-120
 \beta = \frac{-120}{-2}
 \beta =60

 \alpha +60=180
 \alpha =180-60
 \alpha =120

cos (120) = -0,5
cos (60) = 0,5
-0,5 + 0,5 = 0