Problem A. 619. (May 2014)
A. 619. There are given four rays, \(\displaystyle a\), \(\displaystyle b\), \(\displaystyle c\) and \(\displaystyle d\) in space, starting from the same point, laying in a plane \(\displaystyle \varPi\). For an arbitrary acute angle \(\displaystyle \varphi\), rotate \(\displaystyle \varPi\) by angle \(\displaystyle \varphi\) in positive direction around each of the four rays; denote the rotated planes by \(\displaystyle A_\varphi\), \(\displaystyle B_\varphi\), \(\displaystyle \varGamma_\varphi\) and \(\displaystyle \varDelta_\varphi\), respectively. Let \(\displaystyle \varSigma_\varphi\) be the plane through the intersection line of \(\displaystyle A_\varphi\) and \(\displaystyle B_\varphi\), and the intersection line of \(\displaystyle \varGamma_\varphi\) and \(\displaystyle \varDelta_\varphi\). Show that the planes \(\displaystyle \varSigma_\varphi\) share a common line.
(5 pont)
Deadline expired on 10 June 2014.
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