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R = u² Sin 2 Ф / g

So Sin 2Ф1 = Sin 2Ф2

2Ф1 = 2Ф2  or  2Ф1 = 180 - 2Ф2 

Ф1 = Ф2  or  Ф1 = 90 - Ф2 

2 5 2