Bài 1: Áp dụng công thức $\cos 2x=2cos^2{x}-1$ ta có$P=cos^2{a}+cos^2{2a}+cos^2{3a}-1$
$=\frac{cos{2a}+1}{2}+\frac{cos{4a}+1}{2}+\frac{cos{6a}+1}{2}-1$
$=\frac{1}{2}(cos2a+cos 4a+cos6a+1)$
Mà $cos2a+cos6a=2cos4a . cos2a$
$cos4a +1=2cos^2{2a}$
$\Rightarrow P=cos2a(cos4a+cos2a)=2cosa.cos2a.cos3a$