ρ=√2cos(θ-π/4)=cosθ-sinθ
ρ^2=ρcosθ-ρsinθ
x^2+y^2=x-y
将x=1+(4/5)t、y=-1-(3/5)t代入曲线方程得：t^2+7t/5=0
t1=-7/5、t2=0
所求弦长为：|t1-t2|=7/5.
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