FunctionHard

Question

If f(x) = , then (fof) (x) = x, provided that

Options

A.d + a = 0
B.d - a = 0
C.a = b = c = d = 1
D.a = b = 1

Solution

f(x) =
fof(x) =
fof(x) =
fof(x) = = x
on comparing coefficient of both side (a2 + bc) x + (ab + bd) = (ac + cd) x2 + (bc + d2) x
a2 + bc = bc + d2   ⇒   a = d or a = - d
and    ab + bd = 0        ⇒    b = 0 or a = - d
and    ac + cd = 0        ⇒    c = 0 or a = - d
which can be simultaneously true for a = - d

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