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小霞的fans · 發表於 14/10/2008 10:12 PM

f(x)=3x-1 and g(x)=x-2

a) if 2f(a)-3g(a)=5 find the values of a
b)if f(2b)-g(2b)=5 find the value of b

唔明佢up乜=( thx

abc666 · 發表於 14/10/2008 10:34 PM

a) 2 x ( 3a - 1 ) - 3 x ( a - 2 ) = 5

b) [ 3(2b) - 1 ] - [ 2b - 2 ] = 5

sapphire · 發表於 14/10/2008 10:35 PM

f(a) means putting a into x of f(x)
i.e. f(a) = 3a - 1, f(2a) = 3(2a) - 1
similar for g(x)

(a)
2f(a) - 3g(a) = 5
2(3a - 1) - 3(a - 2) = 5
6a - 2 - 3a + 6 = 5
3a = 1
a = 1/3

Now using the similar method, you can find out b, try to do it
ans : b = 1

小霞的fans · 發表於 14/10/2008 10:39 PM

原帖由 abc666 於 14/10/2008 02:34 AM 發表

 登錄/註冊後可看大圖

a) 2 x ( 3a - 1 ) - 3 x ( a - 2 ) = 5

b) [ 3(2b) - 1 ] - [ 2b - 2 ] = 5

i got it =] thx!

但係我想問如果f(3x)=9x^2+6x-3
find f(3)

要將3x=3 x=1 然之後將1代入去 ..

但係呢題點解唔係 2(a/x)-3(a/x)=5 既 ??

[ 本帖最後由小霞的fans 於 14/10/2008 02:45 AM 編輯 ]

sapphire · 發表於 14/10/2008 11:05 PM

原帖由 小霞的fans 於 14/10/2008 10:39 PM 發表

 登錄/註冊後可看大圖

i got it =] thx!

但係我想問如果f(3x)=9x^2+6x-3
find f(3)

要將3x=3 x=1 然之後將1代入去 ..

但係呢題點解唔係 2(a/x)-3(a/x)=5 既 ??

f(3x) = 9x^2 + 6x - 3
f(x) = 9(x / 3) ^ 2 + 6(x / 3) - 3

Always change back the function into the form of f(x).
f(a) means putting x = a of f(x).
Then, to find f(3), just put x = 3 into f(x).

But in this question, the function is f(x) and g(x). So just put x = a.

[ 本帖最後由 sapphire 於 14/10/2008 11:10 PM 編輯 ]

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