How to solve this question? If f(2x)=x^2+3x+9, find the value of 3f(4)-5

mhv12345 said:
If f(2x)=x^2+3x+9, find the value of 3f(4)-5
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You are expected to show some of your own work so that we know how to direct help towards you.

HINT \(\displaystyle f(4)=f(2\cdot 2)=~?\). Now show us what you think.
 
mhv12345 said:
If f(2x)=x^2+3x+9, find the value of 3f(4)-5

Thank you very much!
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Assume u = x/2 → f(u) = ? → f(4) = ? and so on...

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f(2x/2) = x2/2 +3x/2+9

f(x)=0.5x2+1.5x+9

3f(4)-5

2x=4
x=2

f(2) = 0.5(2)2+1.5(2) +9
=37

Right?
 
Simple calculation

f(2x)=x^2+3x+9, find the value of 3f(4)-5

Putting x=2
f(4)=4+6+9=19

So
3X19-5=52
 
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