squaring and adding... 2+2(cos a-b)=0 cos a-b = -1 again, squaring and subtracting, cos 2a + cos 2b + 2cos a+b =0 cos 2a + cos 2b = -2cos a+b but cos a-b = -1 hence a and b differ by angle pi so a=np/2..b=3npi/2 hecne a+b=2npi...cos a+b=1 so cos2a + cos 2b=1 ANS.
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