Advanced mathematics. How do you get the integral limit of a circle?

In polar coordinates, the lower integral limit of dr is the straight line X+Y =1in rectangular coordinates; In polar coordinates, we get rcosθ+rsinθ= 1, r= 1/(cosθ+sinθ), and the upper integral limit of dr is the arc x in rectangular coordinates. +y？ = 1, in polar coordinates, r= 1, which is the upper and lower limit of the figure.

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