The Student Room Group

Composite functions C3

f(x)=ax+b, f(2)=19 and ff(0)=55. Find the possible values of a and b.

So far I worked out ff= (a^2)x + ab
ff(0)= a^2 + ab=55

f(2)= 2a+b=19

I'm not sure what to do next. I tried to solve it simultaneously but I couldn't work it out.
Original post by osayukiigbinoba
f(x)=ax+b, f(2)=19 and ff(0)=55. Find the possible values of a and b.

So far I worked out ff= (a^2)x + ab
ff(0)= a^2 + ab=55

f(2)= 2a+b=19

I'm not sure what to do next. I tried to solve it simultaneously but I couldn't work it out.


Your ff(x) is incorrect.

ff(x) = f(f(x))= af(x)+b
= a(ax+b) +b

= a^2x + ab + b

Then set x=0
Is this for edexcel exam board? If so what past paper question is it? I got two sets of values for A and B
Reply 3
Avatar for yuki11
yuki11
OP
5
Original post by ghostwalker
Your ff(x) is incorrect.

ff(x) = f(f(x))= af(x)+b
= a(ax+b) +b

= a^2x + ab + b

Then set x=0


Oh ok thank you, I'll try that.
Reply 4
Avatar for yuki11
yuki11
OP
5
Original post by JoshBrown97
Is this for edexcel exam board? If so what past paper question is it? I got two sets of values for A and B


It's from an Ocr textbook and it says the answers are a=4, b=11 or a=4.5, b=10
Original post by osayukiigbinoba
It's from an Ocr textbook and it says the answers are a=4, b=11 or a=4.5, b=10


Yes I got them answers. Firstly I solved f(2) = 2a + b (which equalled 19). Then I solved ff(0). f(0) = b then f(b) (as it is ff(0)) equals 55.

Therefore you have 2 equations

2a + b = 19 ----> 1
ab + b = 55 ----> 2

Rearrange to find b. -----> b = 55/(a-1)

Sub the equation we have for b into equation 1. which will turn out to be a quadratic and we factorise for a = 9/2 (4.5) or a = 4. Sub these values back into either equation to locate b. The b values turn out to be b = 10 and b = 11
Reply 6
Avatar for yuki11
yuki11
OP
5
Original post by JoshBrown97
Yes I got them answers. Firstly I solved f(2) = 2a + b (which equalled 19). Then I solved ff(0). f(0) = b then f(b) (as it is ff(0)) equals 55.

Therefore you have 2 equations

2a + b = 19 ----> 1
ab + b = 55 ----> 2

Rearrange to find b. -----> b = 55/(a-1)

Sub the equation we have for b into equation 1. which will turn out to be a quadratic and we factorise for a = 9/2 (4.5) or a = 4. Sub these values back into either equation to locate b. The b values turn out to be b = 10 and b = 11


Sorry for the late reply, the TSR doesn't send me email notifications about posts:angry: often anymore so I just saw this now. Thank you very much for the help.

Quick Reply

Related discussions

Latest

Trending

Trending

Articles for you