Question

cos(a+b) если соsa=0,6, sinb=-8/17

Answer 1

cos(a+b)=cosacosb-sinasinb
cosb=$sqrt{1-sin^2b} = sqrt{1- frac{64}{289} } =frac{15}{17}$
sina=$sqrt{1-cos^2a}= sqrt{1-0.36} =0.8$
cos(a+b)=0.6*$frac{15}{17} -0.8*(- frac{8}{17} )= frac{9}{17} + frac{32}{85} = frac{77}{85}$