91Ó°ÊÓ

Monthly normal rainfall data \((x, y)\) for Portland, Oregon, are \((4,2.47),(7,0.58),(8,1.07),\) where \(x\) represents time in months (with \(x=1\) representing January) and \(y\) represents rainfall in inches. Find the values of \(a, b,\) and \(c\) rounded to 2 decimal places such that the equation \(y=a x^{2}+b x+c\) models this data. According to your model, how much rain should Portland expect during September? (Source: National Climatic Data Center)

Solve each system of equations. $$ \left\\{\begin{array}{l} {\frac{y}{5}=\frac{8-x}{2}} \\ {x=\frac{2 y-8}{3}} \end{array}\right. $$

Solve each system of equations. $$ \left\\{\begin{array}{r} {4 x-7 y=7} \\ {12 x-21 y=24} \end{array}\right. $$

The function \(f(x)=-8.6 x+275\) represents the U.S. average number of monthly calls (sent or received) per wireless subscriber and the function \(f(x)=204.9 x-1217\) represents the average number of text messages (sent or received) per wireless subscriber. For both functions, \(x\) is the number of years since 2000 , and these functions are good for the years \(2006-2010\). Solve the system formed by these functions. Round each coordinate to the nearest whole number.

Solve each system of equations. $$ \left\\{\begin{aligned} 2 x-5 y &=12 \\ -4 x+10 y &=20 \end{aligned}\right. $$

Solve each system of equations. $$ \left\\{\begin{array}{c} {\frac{2}{3} x-\frac{3}{4} y=-1} \\ {-\frac{1}{6} x+\frac{3}{8} y=1} \end{array}\right. $$

Solve each system of equations. $$ \left\\{\begin{array}{l} {\frac{1}{2} x-\frac{1}{3} y=-3} \\ {\frac{1}{8} x+\frac{1}{6} y=0} \end{array}\right. $$

Solve each system of equations. $$ \left\\{\begin{array}{r} {0.7 x-0.2 y=-1.6} \\ {0.2 x-y=-1.4} \end{array}\right. $$

Solve each system of equations. $$ \left\\{\begin{array}{c} {-0.7 x+0.6 y=1.3} \\ {0.5 x-0.3 y=-0.8} \end{array}\right. $$

Solve each system of equations. $$ \left\\{\begin{array}{l} {4 x-1.5 y=10.2} \\ {2 x+7.8 y=-25.68} \end{array}\right. $$