sinα+sinβ=1/2 =>(sina+sinβ)^2=1/4 sin^2a+2sinasinβ+sin^2β=1/4. (1)cosα+cosβ=1/3 =>(cosa+cosβ)^2=1/9 cos^2a+2cosacosβ+cos^2β=1/9(2)(1)+(2) =>2+2(cosacosβ+sinasinβ)=1/4+1/9=13/36. 2(cosaco...