a) cosα-cosβ=-2(sin(α+β)/2)*sin(α-β)/2

sinα-sinβ=2(sin(α-β)/2)*cos(α+β)/2

(cos51°-cos9°)/(sin51°-sin9°)=-2((sin30°)*sin(21°))/2(sin(21°))*cos(30°)=

-2/(2√3)=-√3/3

б) sinα+sinβ=2(sin(α+β)/2)*cos(α-β)/2

(sin11π/72+sin13π/72)-cos(π/72)=2(sin(24π)/(72*2))*cos(π/72)-cos(π/72)=

2(sinπ/6)*cos(π/72)-cos(π/72)=cos(π)/72-cos(π)/72=0