Question

Please help I'm not sure what should I substitute the variable (x) by

Answer 1

From the given table, the quadratic model is given by

`y=1.2x^2+13x+504.3`

which corresponds to option B.

The general quadratic model is given by

`y=Cx^2+Bx+A`

and we need to find the constants A, B and C. They are given by

and

For instance, the variance for x, denoted by S_xx is given by

`S_(x*)=(0-20)^2+(10-20)^2+(20-20)^2+(30-20)^2+(40-20)^2`

where x is the variable which corresponds to the "years since 1970" and the number 20 in each parenthesis is the mean of the this variable, that is

`\bar{x}=(0+10+20+30+40)/(5)=20`

Now, the variance S_xy is given by