Question 945993

{{{cos(120) +cos(30) }}}

 you are supposed to express {{{cos(120)}}} as {{{cos(90 + 30)}}} and change it to single angles with the standard formula

{{{cos(a + b) = cos(a)*cos(b) - sin(a)*sin(b)}}}

{{{cos(90+30) +cos(30)}}} 

={{{cos(90)*cos(30) - sin(90)*sin(30)+cos(30)}}} ........since {{{cos(90)=0}}}, {{{sin(90)=1}}}, we have

={{{0*cos(30) +cos(30) - 1*sin(30)}}}


={{{cos(30) - sin(30)}}}


={{{sqrt(3)/2-1/2}}}