[Toán 10] Lượng giác

L

lp_qt

$P=4(cosa^4+sina^4)−cosa^4$

$=4.[(sin^2a+cos^2a)^2-2.sin^2a.cos^2a]−cosa^4$

$=4.[1-2.(\dfrac{1}{2}.sin2a)^2]−cosa^4$

$=4.[1-\dfrac{1}{2}.sin^22a]−cosa^4$

$=4-2.(\dfrac{1-cos^4a}{2})−cosa^4$

$=3$
 
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