Question

I'm having trouble plotting a quadratic model in the program R. This is my code:

> sales <- c(3.9, 4.3, 5.4, 6.4, 8.1)
> month <- 1:5
> 
> plot(month, sales)
> 
> linear.model <- lm(sales ~ month)
> # summary(linear.model)
> abline(linear.model)
> 
> month2 <- month^2
> quadratic.model <- lm(sales ~ month + month2)
> # summary(quadratic.model)
> 
> predictedmonths <- 6:12
> predictedsales <- predict(quadratic.model, list(month = predictedmonths, month2 = predictedmonths^2))
> plot(month, predictedsales)
Error in xy.coords(x, y, xlabel, ylabel, log) : 
  'x' and 'y' lengths differ

For reference, this is the question I'm trying answer:

The owner of a rapidly expanding business finds that for the first five months of the year, the sales
are $3.9, $4.3, $5.4, $6.4, and $8.1 (thousands). After plotting these figures, the owner speculates
that they follow a quadratic function. Find the least-squares quadratic polynomial fit to these data
and use it to project sales for the remaining seven months of the year.

Answer 1

> sales <- c(3.9, 4.3, 5.4, 6.4, 8.1)
> month <- 1:5
>
> plot(month, sales)
>
> linear.model <- lm(sales ~ month)
> # summary(linear.model)
> abline(linear.model)
>
> month2 <- month^2
> quadratic.model <- lm(sales ~ month + month2)
> # summary(quadratic.model)
>
> predictedmonths <- 6:12
> predictedsales <- predict(quadratic.model, list(month = predictedmonths, month2 = predictedmonths^2))
>
> ## Plot of projected sales for the remaining seven months of the year
> plot(predictedmonths, predictedsales)