Solution :
The formula of cot (A + B) is cotAcotB–1cotB+cotA.
Proof : We have,
cot (A + B) = cos(A+B)sin(A+B)
Using sin (A + B) and cos (A + B) formula,
cot (A + B) = cosAcosB–sinAsinBsinAcosB+cosAsinB
Dividing the numerator and denominator by sin A sin B,
cot (A + B) = cotAcotB–1cotB+cotA