Please help me.

 see proof below

Step-by-step explanation:

Use the Sum/Difference Identity: tan (A + B) = (tanA + tanB)/(*1 - tanA · tanB)

Scratchwork:

tan (50) = tan (40 + 10)

tan 50 (1 - tan 40 · tan 10) = tan 40 + tan 10

tan 50 - tan 50 · tan 40 · tan 10 = tan 40 + tan 10

tan 50 = tan 40 + tan 10 + tan 50 · tan 40 · tan 10

Use the Reciprocal Identity: cot A = 1/ tan --> cot A · tan A = 1

Proof LHS → RHS

LHS:                           tan 50    - tan 40

Substitute tan 50:     tan 40 + tan 10 + (tan 50 · tan 40) · tan 10     - tan 40

Simplify:                     tan 10 + (tan 50 · tan 40) · tan 10

Reciprocal Identity:   tan 10 + tan 10

Simplify:                    2 tan 10

LHS = RHS:  2 tan 10 = 2 tan 10    

the domain of a function is the set of input values, x , for which a function is defined. the domain is shown in the left oval in the picture below. the function provides an output value, f(x) , for each member of the domain.