The graph of a proportional relationship contains the point (8, 4).

1 answer:

6 0

$\bf \qquad \qquad \textit{direct proportional variation}\\\\
\textit{\underline{y} varies directly with \underline{x}}\qquad \qquad y=kx\impliedby 
\begin{array}{llll}
k=constant\ of\\
\qquad variation
\end{array}\\\\
-------------------------------\\\\
(\stackrel{x}{8}~~,~~\stackrel{y}{4})\implies 
\begin{cases}
x=8\\
y=4
\end{cases}\implies 4=k8\implies \cfrac{4}{8}=k\implies \cfrac{1}{2}=k
\\\\\\
\boxed{y=\cfrac{1}{2}x}$

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13.

(a)

$y=-5x^2+21x-3$

for x = 2:

$y=-5(2)^2+21(2)-3=-20+42-3=19$

for x = 5

$y=-5(5)^2+21(5)-3=-125+105-3=-23$

(b)

The graph is:

(c)

Using the graph:

$\begin{gathered} x=1.8,y=18.6 \\ x=0.548,y=7 \\ x=3.65,y=7 \end{gathered}$ $\begin{gathered} x=1.8 \\ y=-5(1.8)^2+21(1.8)-3=-16.2+37.8-3=18.6 \end{gathered}$ $\begin{gathered} y=7 \\ -5x^2+21x-3=7 \\ -5x^2+21x-10=0 \end{gathered}$

Using the quadratic formula:

$\begin{gathered} x=\frac{-b\pm\sqrt[]{b^2-4ac}}{2a} \\ x=\frac{-21\pm\sqrt[]{21^2-4(-5)(-10)}}{2(-5)} \\ x=\frac{-21\pm\sqrt[]{241}}{-10} \\ x=0.54758 \\ or \\ x=3.6524 \end{gathered}$