In \(\triangle \mathrm{ABC}\), the angles \(\mathrm{A}, \mathrm{C}, \mathrm{B}\) are in A.P. Then \(\frac{a+b}{\sqrt{a^{2}-a b+b^{2}}}\) equals to (1) \(\sin \frac{\mathrm{A}}{2}\) (2) \(\frac{1}{2} \cos \frac{\mathrm{A}+\mathrm{B}}{2}\) (3) \(\cos \frac{\mathrm{B}}{2}\) (4) \(2 \cos \left(\frac{A-B}{2}\right)\)

When angles in a triangle are in arithmetic progression (A.P.), this means the difference between consecutive angles is constant. Suppose we have three angles: \( A \), \( B \), and \( C \). If they are in A.P., then we can express them as:
\[ A = B - d \] \[ B = B \] and \[ C = B + d \],
where \( d \) is the common difference. For any triangle, the total sum of internal angles is always \( 180^{\text{o}} \). Given that: \[ (B - d) + B + (B + d) = 180^{\text{o}} \],
simplifying this, we get:
\[ 3B = 180^{\text{o}} \] \[ B = 60^{\text{o}} \].
So, angles \( A \) and \( C \) are given as \( A = 60^{\text{o}} - d \) and \( C = 60^{\text{o}} + d \), respectively. This important property helps us form the triangle's angles precisely based on the given arithmetic progression.

The Law of Cosines is a fundamental rule for solving triangles when you know two sides and the included angle or all three sides. For any triangle with sides \( a \), \( b \), and \( c \) opposite to angles \( A \), \( B \), and \( C \) respectively, it states:
\[ a^2 = b^2 + c^2 - 2bc \cos(A) \] \[ b^2 = a^2 + c^2 - 2ac \cos(B) \] \[ c^2 = a^2 + b^2 - 2ab \cos(C) \].
When angles are in A.P. and \( B = 60^{\text{o}} \), \( \cos(60^{\text{o}}) = \frac{1}{2} \), these equations simplify our calculation. To find the sides \( a \), \( b \), and \( c \) relative to each other, corresponding to their opposite angles, we use these equations. For instance, calculating information about side \( a \) involves substituting the known values of \( b \), \( c \), and angle \( A \). Understanding how these equations interrelate is crucial for solving more complex triangle problems.

Trigonometric identities are equalities involving trigonometric functions that are true for all values of the variables where both sides are defined. They simplify complex trigonometric expressions and are vital in solving different equations and identities. For instance, in the exercise given, we have to simplify and break down:
\[ \frac{a+b}{\sqrt{a^2 - ab + b^2}} \].
Using the relationships between angles and sides defined by trigonometric identities such as: \[ \cos(60^{\text{o}}) = \frac{1}{2} \] and common identities like: \[ \sin^2(x) + \cos^2(x) = 1 \], we make complex terms easier to handle. Substituting the known values and simplifying leads us to:
\[ 2 \cos \left(\frac{A - B}{2}\right) \].
These identities help confirm step-by-step problem-solving to arrive at the final result, ensuring the solution’s correctness.