cos105+cos75 = 2cos((105+75)/2)cos((105-75)/2) = 2cos90cos15=0
cos 105°+ cos 75°=cos(60+45)+cos(30+45)=cos60cos45-sin60sin45 +cos30cos45-sin30sin45=1/2*√2/2-√3/2*√2/2+√3/2*√2/2-1/2*√2/2=0
cos 105°+ cos 75°=cos(180-75)+cos75=-cos75+cos75=0
z/5+m/2=5 z/4-m/3=0.5
2z+5m/10=5 3z-4m/12=0.5
2z+5m=50 3z-4m=6
z=25-2.5m z=4m+2
25-2.5m=4m+6/3
75-7.5m=4m+6
69=11.5m
m=6 тогда z=10