The value of (1 − tan15°)/(1 + tan15°) is:

Since tan45° = 1, the expression is (tan45° − tan15°)/(1 + tan45°·tan15°) = tan(45° − 15°) = tan30° = 1/√3 ≈ 0.577. Numerically: (1 − 0.2679)/(1 + 0.2679) = 0.7321/1.2679 = 0.577. The value 2 − √3 is tan15° itself.