三角比數......(10點)

5sinθ^2 + 3sinθcosθ = 1
5sinθ^2 + 3sinθcosθ = sinθ^2 + cosθ^2
4sinθ^2 + 3sinθcosθ - cosθ^2 = 0 ...... (i)
(sinθ - cosθ) (4sinθ + cosθ) = 0
sinθ = cosθ or 4sinθ = -cosθ
tanθ = 1 or -1/4
θ = 45 or 165.96

你可以 Let x = sinθ, y = cosθ, x^2 + y^2 = 1
條式就會變 5x^2 + 3xy = 1
5x^2 + 3xy - y^2 = 0 ...... [Same as (i)]
...
x = y or 4x = -y
咁就可以代番sinθ同cosθ落去計

2007-11-25 20:56:32 補充：
希望下面果d唔好抄la...