c: \(=-\dfrac{1}{2}\cdot\left[cos\left(4a+10a\right)-cos\left(4a-10a\right)\right]+\dfrac{1}{2}\cdot\left[cos\left(11a+3a\right)-cos\left(11a-3a\right)\right]+\dfrac{1}{2}\cdot\left[cos\left(7a+a\right)-cos\left(7a-a\right)\right]\)
\(=\dfrac{-1}{2}\cdot cos14a+\dfrac{1}{2}\cdot cos\left(-6a\right)+\dfrac{1}{2}\cdot cos14a-\dfrac{1}{2}\cdot cos8a+\dfrac{1}{2}\cdot cos8a-\dfrac{1}{2}\cdot cos6a\)
=0
d: Tham khảo:
=1-cos^2A
=sin^2A
Đúng 0
Bình luận (0)
