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Solutions
f(a)f(b) = f(a) + f(b) + f(ab) - 2
Put a = b = 1.
[f(1)]2 =3f(1)−2 ⇒ f(1) = 1 (or) 2.
Let's assume f(1) = 1
Now, put b = 1.
f(a) = 2f(a) - 1
⇒ f(a) = 1 ⇒ For all values of a, f(a) = 1.
This is false because f(4) = 17.
⇒ f(1) = 2 is the correct value.
Now put b = 1/a
f(a)f(1/a) = f(a) + f(1/a) + 2 - 2
⇒ f(a)f(1/a) = f(a) + f(1/a)
So taking RHS terms to LHS and adding 1 to both sides we get
f(a)f(1/a) - f(a) - f(1/a) +1 = 1
(f(a) - 1) (f(1/a)-1) = 1
Let g(x) = f(x)-1
So g(x)*g(1/x) = 1
So g(x) is of the form ±xn
So f(x) is of the form ±xn +1.
f(a) = an +1 satisfies the above condition.
−4n +1 = 17 ⇒ -4n−4 n = 16, which is not possible.
4a +1 = 17 ⇒ n = 2
⇒ f(a) = a2 + 1a 2 +1
⇒ f(7) = 72 + 1 = 50