Functions f and g are both defined for \(x \in \mathbb{R}\) and are given by
\(f(x) = x^2 - 4x + 9,\)
\(g(x) = 2x^2 + 4x + 12.\)
(a) Express \(f(x)\) in the form \((x-a)^2 + b.\) [1]
(b) Express \(g(x)\) in the form \(2[(x+c)^2 + d].\) [2]
(c) Express \(g(x)\) in the form \(kf(x+h),\) where \(k\) and \(h\) are integers. [1]
(d) Describe fully the two transformations that have been combined to transform the graph of \(y = f(x)\) to the graph of \(y = g(x).\) [4]
Solution
(a) To express \(f(x) = x^2 - 4x + 9\) in the form \((x-a)^2 + b\), complete the square:
\(f(x) = (x-2)^2 + 5\)
(b) To express \(g(x) = 2x^2 + 4x + 12\) in the form \(2[(x+c)^2 + d]\), first factor out the 2:
\(g(x) = 2(x^2 + 2x + 6)\)
Complete the square for \(x^2 + 2x + 6\):
\(x^2 + 2x + 6 = (x+1)^2 + 5\)
Thus, \(g(x) = 2[(x+1)^2 + 5]\)
(c) Express \(g(x)\) in the form \(kf(x+h)\):
\(g(x) = 2(x^2 + 2x + 6) = 2[(x+3)^2 + 5]\)
So, \(g(x) = 2f(x+3)\) where \(k=2\) and \(h=3\)
(d) The transformations from \(y = f(x)\) to \(y = g(x)\) are:
- Translation by \(\begin{pmatrix} -3 \\ 0 \end{pmatrix}\)
- Stretch in the y-direction by a factor of 2
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