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How to know the integral interval of curve integral in the sixth edition of advanced mathematics?
Rou can be positive or negative, but for convenience, rou is taken as the length of this line segment, which is generally positive.

θ is the polar angle, which is defined as the counterclockwise rotation angle from the positive direction of the X axis in the polar coordinate system. For example, if the X axis is rotated 90 degrees counterclockwise to get the positive direction of the Y axis, then the polar angle rou of the point in the positive direction of the Y axis is π/2. In the second picture, you can take a ruler, one end of which leans against the O point, and start to rotate from the positive direction of the X axis, counterclockwise is positive, clockwise is negative, until the ruler and the circle do not intersect, so,