tan=2
sin/cos=2
sin^2+cos^2=1
sin=2cos
sin^2=4cos^2
cos^2+4cos^2=1
5cos^2=1
cos^2=1/5
2cos^2=2/5
3sin^2=12cos^2=12/5
3sin^2θ－2cos^2θ＝12/5-2/5=10/5=2
3sin^2θ－2cos^2θ＝2.