All Junior Cert Mathematics posts
• #### Anyone get Question 4 in the maths HL paper? SStudent

Let f (x) = 3x + 5, for x ∈ ℝ.

(a) Find the value of f (7).

(b) Write f (k ) in terms of k.

(c) Using your answer to part (b), or otherwise, find the value of k for which f (k ) = k.

1. #### Yasmine146

For part a put 7 in for x and solve it: f(7)=3(7) + 5

f(7)=26

I think i got b and c wrong :] It seemed too easy lol.

2. #### SStudent

i got a but not b and c

3. #### Atom

f(x) is a function. To keep things simple, you sub the 7 into the slot of X. Therefore;

A) f(7) = 3(7) + 5, f(7) = 26

B) Using what I said above, you slot K into the position of X. Therefore, 3x + 5 becomes 3k + 5

C) Because we know what f(k) is, 3k + 5, we can leave that equal to k. Therefore; 3k + 5 = k, moving things around it becomes 3k - k = -5. In the end K works out to be -5/2 or -2.5.

4. #### frans

i but down for (B) : f(k)=3k + 5

5. #### SStudent

ugh looking back i could have probably figured that out if it wasn't an exam...ugh

6. #### Nara

I still don't get (c)