\(=\frac{cos36-sin\left(180+54\right)}{sin\left(180-36\right)-cos\left(90+36\right)}.cos54=\frac{cos36+sin54}{sin36+sin36}.cos54\)
\(=\frac{cos36+cos36}{2sin36}.sin36=cos36\)
\(=\frac{cos36-sin\left(180+54\right)}{sin\left(180-36\right)-cos\left(90+36\right)}.cos54=\frac{cos36+sin54}{sin36+sin36}.cos54\)
\(=\frac{cos36+cos36}{2sin36}.sin36=cos36\)
Chứng minh
\(\frac{1+cosx+cos2x+cos3x}{2cos^2x+cosx-1}\)=2cosx
\(\frac{cos4a\cdot tan2a-sin4a}{cos4a\cdot cot2a+sin4a}\)=-tan22a
Giúp mình vs! Mình đang cần gấp :((
Rút gọn
Rút gọn
rút gọn: \(\dfrac{1-2sin^2x}{cos2x-sin2x}\)
Rút gọn: \(\dfrac{sina+cosa-1}{sina-cosa+1}\)
Rút gọn biểu thức: 3(sin8x - cos8x) + 4(cos6x - 2sin6x) + 6sin4x
rút gọn
\(\dfrac{sin2a+1}{cos2a}-\dfrac{1-sin2a}{sina-cosb}\)
Rút gọn biểu thức A= (1+cotx)sin^3x+(1+tanx)cos^3x
Rút gọn :\(\dfrac{cos2\alpha+cos4\alpha+cos6\alpha}{sin2\alpha+sin4\alpha+sin6\alpha}\)