Question

Se α + β= 180° e α - β = 60° , determine o valor de cos α + cos β. 

Answer 1

$\alpha + \beta =180$
$\alpha - \beta =60$
$\alpha =180- \beta$
$180-\beta - \beta =60$
$-\beta - \beta =60-180$
$-2 \beta =-120$
$\beta = \frac{-120}{-2}$
$\beta =60$

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

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