cos(pi/4+ a)^2 – cos(pi/4- a)^2 rút gọn biểu thức 01/10/2021 Bởi Adalynn cos(pi/4+ a)^2 – cos(pi/4- a)^2 rút gọn biểu thức
$\cos^2(\dfrac{\pi}{4}+a) -\cos^2(\dfrac{\pi}{4}-a)$ $=\dfrac{1+\cos(\dfrac{\pi}{2}+2a)}{2}-\dfrac{1+\cos(\dfrac{\pi}{2}-2a)}{2}$ $=\dfrac{1-\sin2a -1-\sin2a}{2}$ $= -\sin2a$ Bình luận
$\cos^2(\dfrac{\pi}{4}+a) -\cos^2(\dfrac{\pi}{4}-a)$
$=\dfrac{1+\cos(\dfrac{\pi}{2}+2a)}{2}-\dfrac{1+\cos(\dfrac{\pi}{2}-2a)}{2}$
$=\dfrac{1-\sin2a -1-\sin2a}{2}$
$= -\sin2a$