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20 1 1 Tianjin college entrance examination 14 can be solved in what way? Thank you.
Draw a picture first, and then make a plane rectangular coordinate system with C as the origin. Let CD be m long and CP be a long, that is, A(-2, m-a)P(0, a).

B(- 1, 0), then PA=(-2, m-a)PB=(- 1, -a), PA+3PB=(-5, m-4a)|PA+3PB| square =25+(m-4a).