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Suppose `f` is a real function satisfying `f(x+f(x))=4f(x)a n df(1)=4.` Then the value of `f(21)` is `16` `21` `64` `105`

A

16

B

64

C

4

D

44

Text Solution

Verified by Experts

The correct Answer is:
B
`f(x+f(x))=4f(x) and f(1)=4`
Put x = 1, then
`f(1+f(1))=4f(1)`
`rArr" "f(5)=16`
Put x = 5, then
`f(5+f(5))=4f(5)`
`rArr" "f(21)=64`
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