Correct solutions were submitted by Bradley University community members Limin Liang, Monie Teslik, Mike Nowak, Nick Spence, Luke Bokewitz, Peter Petrany. Other correct solutions were submitted by Lou Cairoli, Bryan Fluhrer, Iñigo Picaza, Francesc Suñol i Esquirol, Heather Uchima, Ahron Teitelman, Juan Carlos Marivela, Thiry Yves, Bill Webb, George Davis, Lisa Schechner, Paul Botham, Dan Dima, Modechai Martin Rappaport.
The folded angle measures 90º. In the figure on the left, the angles labeled a and those labeled b are equal. Hence 2a + 2b = 180º, from which the result follows.
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