докажите,что четырехугольник у которого две стороны паралельны и углы,прилежащие к одной из этих сторон ,прямые, является прямоугольником 

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On a cold day in 1942, inside a Nazi concentration camp, a lonely, young boy looks barbed wire and sees a young girl passing by. She also notices him. In an effort to give expression to her feelings, she throws a red apple over the fence-a sign of life, hope, and love. The young boy bends over, picks up the apple. An expression of joy touches his eyes, a ray of light appears in the darkness.
The following day, thinking he is crazy for even dreaming of seeing this young girl again, he looks out beyond the fence, hoping. On the other side of the barbed wire, the young girl looks forward to seeing again tragic figure who moved her so much. She comes again with an apple in hand.
Despite cold winter winds and chilling air, the two hearts are warmed and the boy gets an apple again. The scene is repeated for several days. The two young spirits on opposite sides of the fence look forward to seeing each other, just for a moment and just to exchange a few words.
At the last of these momentary meetings, the young boy greets his sweet friend with a frown and says, “Tomorrow don’t bring me an apple, I will not be here. They are sending me to another camp”. The young boy walks away, too heartbroken to look back. 

[tex](2x-y^{2})y'=2y[/tex]

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