Given: sinθ−cosθsinθ+cosθ=3
Cross multiply to eliminate the fraction:
sinθ+cosθ=3(sinθ−cosθ)
Expand and collect like terms:
sinθ+cosθ=3sinθ−3cosθ
sinθ−3sinθ=−cosθ−3cosθ
−2sinθ=−4cosθ
sinθ=2cosθ
Use the Pythagorean identity sin2θ+cos2θ=1:
(2cosθ)2+cos2θ=1
4cos2θ+cos2θ=1
5cos2θ=1
cos2θ=51
Therefore: sin2θ=4cos2θ=54
Apply the difference of squares formula a4−b4=(a2+b2)(a2−b2):
sin4θ−cos4θ=(sin2θ+cos2θ)(sin2θ−cos2θ)
Substitute our values:
=(1)(sin2θ−cos2θ)
=54−51
=53