Hệ thức lượng giác

L

lp_qt

Câu 2

$cos^6a+sin^6a$

$=(sin^2a+cos^2a)^3-3.(sin^2a+cos^2a).sin^2a.cos^2a$

$=1-3.(\dfrac{1}{2}.sin2a)^2$

$=1-\dfrac{3}{4}.sin^22a$

$=1-\dfrac{3}{4}(\dfrac{1-cos^4a}{2})$

$=\dfrac{5}{8}+\dfrac{3}{8}.cos^4a$
 
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