x^2=1+4sinθcosθ+3cos^2θ

2sin2θ+3/2cos2θ＝4sinθcosθ+3/2(2cos^2θ-1)
＝4sinθcosθ+3cos^2θ-3/2
＝x^2-1-3/2＝x^2-5/2