document.write( "Question 692844: A pleasant day Sir/ Madam, my daughter has this problem in geometry and I am not good in geometric proofs, can you please help me?\r
\n" ); document.write( "\n" ); document.write( "the illustration or drawing is a rectangle inscribed in a circle
\n" ); document.write( "Given: Quadrilateral ABCD is a rectangle
\n" ); document.write( "Prove: line segment AC and line segment BD are diameters of the circle\r
\n" ); document.write( "\n" ); document.write( "Thank you very much for anyone who can help me!
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Algebra.Com's Answer #427124 by MathLover1(20850)\"\" \"About 
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\n" ); document.write( "first draw the picture:\r
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\n" ); document.write( "\n" ); document.write( "next, we will need to prove this:
\n" ); document.write( "theorem: Diagonal of any rectangle inscribed in a circle is a diameter of the circle. (\r
\n" ); document.write( "\n" ); document.write( "This is essentially the converse of Thales' theorem.\r
\n" ); document.write( "\n" ); document.write( "Let A, B, C and D be the vertices of a rectangle inscribed in a circle and let AC and BD be the diagonals of this rectangle. \r
\n" ); document.write( "\n" ); document.write( "We can now focus on any one of the four triangles: \r
\n" ); document.write( "\n" ); document.write( "\"ABC\", \"BCD\", \"CDA\" and \"DAB\"\r
\n" ); document.write( "\n" ); document.write( "Let's take \"ABC\":\r
\n" ); document.write( "\n" ); document.write( "\"AC\" side of this triangle is the \"diagonal\" of the rectangle\r
\n" ); document.write( "\n" ); document.write( "\"AB\" and \"BC\" are two sides of the rectangle, which potentially may be of different length (but we will prove they must be the \"same\" if the area of the inscribed rectangle is maximized).
\n" ); document.write( "The angle at \"B\" is the right angle since it is one of the angles of the rectangle, which by definition has four right angles.\r
\n" ); document.write( "\n" ); document.write( "Hence, looking at the triangle\"+ABC\", we can see that this is a \"right-angled\" triangle inscribed in the circle.\r
\n" ); document.write( "\n" ); document.write( "Therefore to prove (1), we need to show that side\"+AC\" of any such triangle must be a diameter of circle.\r
\n" ); document.write( "\n" ); document.write( "Proof: \r
\n" ); document.write( "\n" ); document.write( "Choose any three points \"A\", \"B\" and \"C\" on the circle and connect these points to make a triangle \"ABC\".\r
\n" ); document.write( "\n" ); document.write( "Let's suppose that the claim is that the angle at \"B\" is right angle.\r
\n" ); document.write( "\n" ); document.write( "We will show that if this is true then it must follow that \"AC\" is a \"diameter\" of the \"circle\".
\n" ); document.write( "Connect the center of the circle (\"O\") with each of the vertices of the triangle creating the segments \"OA\", \"OB\" and \"OC\".\r
\n" ); document.write( "\n" ); document.write( "Let's call the angle defined by the path \"OAB\" as \"alpha\" and the angle defined by the path \"OCB\" as \"beta\".\r
\n" ); document.write( "\n" ); document.write( "Since \"OA\", \"OC+\"and\"+OB\" are all of \"equal\" length (equal to the length of the radius of the circle ) then \"all\"\"+three\"\"+inner\"\"+triangles\" (\"OAB\", \"OAC\" and \"OBC\") are \"isosceles\".\r
\n" ); document.write( "\n" ); document.write( "We will next want to find the angle between \"OA\" and \"OC+\"(the angle at \"O\" made out by segments \"OA\" and \"OC\") using only angles \"alpha\" and \"beta\"as given.
\n" ); document.write( "The angle between \"OA\" and \"OB\" is equal to \"180+-+2alpha\"° (due to the fact that \"OAB\" is \"isosceles\" and the fact that the sum of all three internal angles in a triangle sum to \"180\"°).\r
\n" ); document.write( "\n" ); document.write( "Similarly, the angle between \"OB\" and \"OC\" is equal to \"180+-+2beta\"°.\r
\n" ); document.write( "\n" ); document.write( "Finally, since all the three angles at \"O\" add up to \"360\"° (full circle), it follows that the angle between \"OA\" and \"OC\" is equal to \"360+-+%28180+-+alpha%29+%96+%28180+-+beta%29+=+2%28alpha%2Bbeta%29\".\r
\n" ); document.write( "\n" ); document.write( "However, the angle at \"B\" of the original triangle \"ABC\" is equal to \"%28alpha%2Bbeta%29\" (follows from the fact that \"OAB\" and \"OBC\" triangles are \"isosceles\").\r
\n" ); document.write( "\n" ); document.write( "This angle was claimed to be \"90\"° and therefore the angle between \"OA\" and \"OC\" is equal to \"180\"°.\r
\n" ); document.write( "\n" ); document.write( "This means that the points \"A\" and \"C\" and the \"center\" of the circle (\"O\") are \"collinear\".\r
\n" ); document.write( "\n" ); document.write( "In other words, the \"center\" of the circle is \"lying\" on the straight line segment \"AC\", which in turn means that \"AC\"\"+must\" be a \"diameter+\"of the\"+circle\".\r
\n" ); document.write( "\n" ); document.write( "So, we can conclude that \"each\"\"+diagonal\" of the rectangle inscribed in a circle \"must\" be a circles \"diameter\".\r
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