✔ 最佳答案
tan27° = p
Sketch a right-angled triangle with angle A = 27°.
For tan27° = p, let p be the opposite side and 1 be the adjacent side.
By Pythagorean theorem :
The hypotenuse =√(p² + 1)
Hence, sin27° = (opposite side)/hypotenuse = p/√(p² + 1)
and cos27° = (adjacent side)/hypotenuse = 1/√(p² + 1)
sin63°
= cos(90° - 63°)
= cos27°
= 1/√(p² + 1)
cos(-126°)
= cos126°
= cos(180° - 54°)
= - cos(54°)
= - cos(2×27°)
= - (cos²27° - sin²27°)
= - {[1/√(p² + 1)]² - [p/√(p² + 1)]²}
= [p²/(p² + 1)] - [1/(p² + 1)]
= (p² - 1)/(p² + 1)
cos126°
= (p² - 1)/(p² + 1)
sin126°
= √{1 - cos²126°}
= √{1 - [(p² - 1)/(p² + 1)]²}
= √{[(p² + 1)/(p² + 1)]² - [(p² - 1)/(p² + 1)]²}
= √{[(p² + 1)² - (p² - 1)]/(p² + 1)²}
= √{[p⁴ + 2p² + 1 - p⁴ + 2p² - 1]/(p² + 1)²}
= 2p/(p² + 1)
cos18°
= cos(270° - 252°)
= - sin(252°)
= - sin(2×126°)
= - 2 sin126° cos126°
= - 2 [2p/(p² + 1)] [(p² - 1)/(p² + 1)]
= -4p(p² - 1)/(p² + 1)²
= p(1 - p²)/(p² + 1)²