1.若a∥b,则2sinθ=cosθ-2sinθ4sinθ=c0sθ tanθ=1/42.0＜θ＜π,|a|=|b|,sin^2θ+(cosθ-2sinθ)^2=5,sin^2 θ-sinθcosθ=1,sin(2θ+π/4)=√2/2 ,θ的值为π/4.【【不清楚,再问；满意, 请采纳!祝你好运...从sin^2 θ-sinθcosθ=1,到sin(2θ+π/4)=√2/2 ,θ的值为π/4.是怎么得到的，麻烦再写详细一点，谢谢！sin^2 θ-sinθcosθ=11/2（1-cos2θ)-1/2 sin2θ=1cos2θ+sin2θ=-1√2sin(2θ+π/4)=-1/2sin(2θ+π/4)=-√2/20＜θ＜πθ=π/2 或θ=3π/4（注意已更正！！）从cos2θ+sin2θ=-1，到√2sin(2θ+π/4)=-1/2是怎么得到的呢？两边同除√2，√2sin(2θ+π/4)=-1我懂了，太谢谢了