Let \[f_n(x)=(2+(-2)^n)x^2+(n+3)x+n^2\] where \(n\) is a positive integer and \(x\) is any real number.
- Write down \(f_3(x)\).
Find the maximum value of \(f_3(x)\).
The maximum value is therefore \(21/2\), achieved at \(x=1/2\).
For what values of \(n\) does \(f_n(x)\) have a maximum value (as \(x\) varies)? [Note you are not being asked to calculate the value of this maximum].
The graph of \(y = f_n(x)\) will be a parabola – it will have a maximum if it points upwards.
So for \(f_n\) to have a maximum we require the coefficient of \(x^2\) in \(f_n\) to be negative.
Consider \(2 + (-2)^n\) – this will be negative exactly when \(n\) is an odd number greater than \(1\).
- Write down \(f_1(x)\). Calculate \(f_1(f_1(x))\) and \(f_1(f_1(f_1(x)))\).
Find an expression, simplified as much as possible, for \[f_1(f_1(f_1(...f_1(x))))\] where \(f_1\) is applied \(k\) times. [Here \(k\) is a positive integer.]
We have \[f_1(x)=4(4(...4(4x+1)+1...)+1)+1\] The coefficient of \(x\) is \(4^k\). The constant term is \[1+4+ ... +4^{k-1}.\] This is a geometric progression with common ratio \(4\), so its sum is \[1 \times \frac{4^k-1}{4-1}=\frac{4^k-1}{3}\] So \(f_1^k(x)=4^kx+\dfrac{4^k-1}{3}\).
- Write down \(f_2(x)\).
\(f_2(x)=6x^2+5x+4\).
If a question says ‘Write down…’ then that is just what we do — no justification is required.
The function \[f_2(f_2(f_2(...f_2(x)))),\] where \(f_2\) is applied \(k\) times, is a polynomial in \(x\). What is the degree of this polynomial?
Every time we apply \(f_2\) to a polynomial, the degree of the polynomial is doubled. So \(f_2^k\) is a polynomial of degree \(2^k\).
