Chứng minh rằng:cos6a+sin6a=1-3cos2a.sin2a cos6a là cos 6 bình a

${\cos}^6a+{\sin}^6a=1-3{\cos}^2a{\sin}^2a$

$VT={\cos}^6a+{\sin}^6a$

$=({\cos}^2a+{\sin}^2a)^3-3{\cos}^4a{\sin}^2a-3{\cos}^2a{\sin}^4a$

$=1-3{\cos}^2a{\sin}^2a({\cos}^2a+{\sin}^2a)$

$=1-3{\cos}^2a{\sin}^2a$

$=VP$