cosA + cosB = 2cos[(A+B)/2]cos[(A-B)/2] from A+B = 120° (A+B)/2 = 60° cos[(A+B)/2] = cos 60° = 1/2 so in the above equation.... 1/√2 = 2(1/2)cos[(A-B)/2] cos[(A-B)/2] = 2(1/√2) / (1/2) = 1/√2
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