Given:
The American marriage rate R, x years after 1960 is defined by the function
[tex]R(x)=-0.0065x^2 +0.23x + 8.47[/tex]
To find:
The year in which the marriage rate is the highest.
Solution:
We have,
[tex]R(x)=-0.0065x^2 +0.23x + 8.47[/tex]
It is a quadratic function with negative leading coefficient. So, it is a downward parabola and vertex of downward parabola is maximum.
If a quadratic function is [tex]f(x)=ax^2+bx+c[/tex], then
[tex]Vertex=\left(\dfrac{-b}{2a},f(\dfrac{-b}{2a})\right)[/tex]
In the given function,
[tex]a=-0.0065,b=0.23,c=8.47[/tex]
Now, x-coordinate of vertex is
[tex]-\dfrac{b}{2a}=-\dfrac{0.23}{2(-0.0065)}[/tex]
[tex]-\dfrac{b}{2a}=-\dfrac{0.23}{-0.013}[/tex]
[tex]-\dfrac{b}{2a}\approx 17.69[/tex]
It means the marriage rate is highest after 17 years of 1960.
[tex]1960+17=1977[/tex]
Therefore, the marriage rate is highest in year 1977.
