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var pantalla = document.querySelector ("canvas");
var pincel = pantalla.getContext("2d");
pincel.fillStyle = "lightgrey";
pincel.fillRect (0,0,450,450);
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pincel.beginPath ();
pincel.moveTo(50,50);
pincel.lineTo(50,400);
pincel.lineTo(400,400);
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pincel.fillStyle = "lightgrey";
pincel.beginPath ();
pincel.moveTo(100,175);
pincel.lineTo(100,350);
pincel.lineTo(275,350);
pincel.fill();
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