=什么?,假设是a
先化成ρsin(θ+π/4)=√2/2*ρsinθ+√2/2*ρcosθ-a=0
所以距离d=|√2/2*2sin7π/4+√2/2*2cos7π/4-a|/√(1/2+1/2)
=|a|