tanθ=3/4cos²θ=1/sec²θ=1/（1+tan²θ）=16/25θ为第三象限角,cosθ＜0所以cosθ=-4/5sinθ=tanθcosθ=-3/5cos（θ-π/4）=cosθcosπ/4+sinθsinπ/4=（-4/5×√2/2）+（-3/5×√2/2）=-7√2/10...