$A=sin^6 a+cos^6 a –sin^2 a$
$=(sin^2a)^3+(cos^2a)^3-sin^2a$
$=(sin^2a+cos^2a)(sin^4a-sin^2a.cos^2a+cos^4a)-sin^2a$
$=sin^4-sin^2a.cos^2a+cos^4a-sin^2a$
$=(sin^4a-2.sin^2a.cos^2a+cos^4a)+sin^2a.cos^2a-sin^2a$
$=(sin^2a-cos^2a)^2+sin^2a(cos^2a-1)$
$=(\dfrac{1}{4})^2-sin^4a$
Tớ nghĩ đc tới đây thôi à !!
$A=sin^6 a+cos^6 a –sin^2 a$
$=(sin^2a)^3+(cos^2a)^3-sin^2a$
$=(sin^2a+cos^2a)(sin^4a-sin^2a.cos^2a+cos^4a)-sin^2a$
$=sin^4-sin^2a.cos^2a+cos^4a-sin^2a$
$=(sin^4a-2.sin^2a.cos^2a+cos^4a)+sin^2a.cos^2a-sin^2a$
$=(sin^2a-cos^2a)^2+sin^2a(cos^2a-1)$
$=(\dfrac{1}{4})^2-sin^4a$
Tớ nghĩ đc tới đây thôi à !!