sinθ=4/5
cosθ=3/5
tanθ=4/3=2tanθ/2/1-tan²θ/2
tanθ/2=x
2=3x/(1-x²)
3x=2-2x²
2x²+3x-2=0
1 2
2 -1
(x+2)(2x-1)=0
tanθ/2=x=1/2