Given data about sale of book as follow.

#To know current directory.
getwd()
## [1] "D:/BUKU UGI/Buku R Ugi dan Erwin/Memasang Kurva Pendekatan Kuadratik terhadap Data"
#Set to the current directory
setwd("D:/BUKU UGI/Buku R Ugi dan Erwin/Memasang Kurva Pendekatan Kuadratik terhadap Data")

#Read or open data "sale of book.csv" in R
data=read.csv("sale of book.csv")

#Show the data
data
##   Year Sale
## 1    1   35
## 2    2   26
## 3    3   19
## 4    4    6
## 5    5    1
## 6    6    3
## 7    7    5
## 8    8   12
## 9    9   17
#Plot data in graphic
attach(data)
plot(Year, Sale, main="Scatterplot", xlab="Year ", ylab="Number of Book That Is Sold", pch=19)

#Fit a quadratic curve
fit2<-lm(Sale~poly(Year,2,raw=TRUE))
summary(fit2)
## 
## Call:
## lm(formula = Sale ~ poly(Year, 2, raw = TRUE))
## 
## Residuals:
##    Min     1Q Median     3Q    Max 
## -2.987 -1.872 -0.039  1.546  4.305 
## 
## Coefficients:
##                            Estimate Std. Error t value Pr(>|t|)    
## (Intercept)                 52.7857     3.3998  15.526 4.52e-06 ***
## poly(Year, 2, raw = TRUE)1 -17.1028     1.5611 -10.956 3.43e-05 ***
## poly(Year, 2, raw = TRUE)2   1.4686     0.1522   9.646 7.11e-05 ***
## ---
## Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
## 
## Residual standard error: 2.672 on 6 degrees of freedom
## Multiple R-squared:  0.9595, Adjusted R-squared:  0.946 
## F-statistic: 71.07 on 2 and 6 DF,  p-value: 6.645e-05
#Show coefficient of regression
fit2$coefficient[1]
## (Intercept) 
##    52.78571
fit2$coefficient[2]
## poly(Year, 2, raw = TRUE)1 
##                  -17.10281
fit2$coefficient[3]
## poly(Year, 2, raw = TRUE)2 
##                   1.468615
#Count value of sale estimation
quadratic = fit2$coefficient[3]*Year^2 + fit2$coefficient[2]*Year + fit2$coefficient[1]
quadratic
## [1] 37.151515 24.454545 14.694805  7.872294  3.987013  3.038961  5.028139
## [8]  9.954545 17.818182
#Fit quadratic curve
plot(Year, Sale, main="Scatterplot", 
xlab="Year ", ylab="Number of Book That Is Sold ", pch=19)
par(new = TRUE)
lines(Year,quadratic, col="red")

Hopefully, this explanation is useful for us ^_^ ~Prana Ugi~