∵sinθ+cosθ=1/2∴(sinθ+cosθ)²=1/4∴sin²θ+cos²θ+2sinθcosθ=1/4又∵sin²θ+cos²θ=1,2sinθcosθ=sin2θ,∴1+sin2θ=1/4∴sin2θ=-3/4