The function $f(x)$ satisfies the functional equation $3f(x) + 2f\left(\dfrac{x+59}{x-1}\right) = 10x + 30$ for all real $x \neq 1$. The value of $f(7)$ is:
Step-by-Step Solution
Key Concept: Solving functional equations by substituting specific values to create a system of equations.
Step 1: Set up the first equation by substituting $x = 7$ into the functional equation.
We substitute $x = 7$ into the given functional equation $3f(x) + 2f\left(\frac{x+59}{x-1}\right) = 10x + 30$.
First, calculate the argument of the second function:
$$\frac{7+59}{7-1} = \frac{66}{6} = 11$$
Substituting into the functional equation:
$$3f(7) + 2f(11) = 10(7) + 30 = 100 \quad \text{...(1)}$$
Step 2: Set up the second equation by substituting $x = 11$ into the functional equation.
We substitute $x = 11$ into the functional equation.
First, calculate the argument of the second function:
$$\frac{11+59}{11-1} = \frac{70}{10} = 7$$
Substituting into the functional equation:
$$3f(11) + 2f(7) = 10(11) + 30 = 140 \quad \text{...(2)}$$
Step 3: Express $f(11)$ in terms of $f(7)$ using equation (1).
From equation (1):
$$3f(7) + 2f(11) = 100$$
$$2f(11) = 100 - 3f(7)$$
$$f(11) = 50 - 1.5f(7)$$
Step 4: Substitute the expression for $f(11)$ into equation (2).
Substituting $f(11) = 50 - 1.5f(7)$ into equation (2):
$$3\left(50 - 1.5f(7)\right) + 2f(7) = 140$$
$$150 - 4.5f(7) + 2f(7) = 140$$
$$150 - 2.5f(7) = 140$$
Step 5: Solve for $f(7)$.
$$-2.5f(7) = 140 - 150$$
$$-2.5f(7) = -10$$
$$f(7) = \frac{-10}{-2.5} = 4$$
**Final Answer:** The value of $f(7) = 4$, which corresponds to **Option 2**.
Correct Answer: 2