Найти cos(a+b), если sina=0.6 и tgb=0,8

Решите задачу:

$cosa*cosb-sina*sinb\\ tgb=\frac{sinb}{cosb}=0.8\\ \frac{sinb}{\sqrt{1-sin^2b}}=0.8\\ \frac{sin^2b}{1-sin^2b}=0.8^2\\ sin^2b=0.8^2-0.8^2*sin^2b\\ 1.64sin^2b=0.8^2\\ sin^2b=\frac{0.64}{1.64}\\ sinb=+/-\frac{0.8}{\sqrt{1.64}}\\ cosb=\sqrt{1-\frac{0.8^2}{1.64}}=\frac{1}{\sqrt{1.64}}\\ cosa=\sqrt{1-0.6^2}=0.8\\ cos(a+b)=0.8*0.6-\frac{0.8}{1.64}=0.48-\frac{1}{2.05}=-\frac{0.016}{2.05}$