bai tap chuong 1

##bai1
set.seed(123)
x <- rnorm(100)
x

##   [1] -0.560475647 -0.230177489  1.558708314  0.070508391  0.129287735
##   [6]  1.715064987  0.460916206 -1.265061235 -0.686852852 -0.445661970
##  [11]  1.224081797  0.359813827  0.400771451  0.110682716 -0.555841135
##  [16]  1.786913137  0.497850478 -1.966617157  0.701355902 -0.472791408
##  [21] -1.067823706 -0.217974915 -1.026004448 -0.728891229 -0.625039268
##  [26] -1.686693311  0.837787044  0.153373118 -1.138136937  1.253814921
##  [31]  0.426464221 -0.295071483  0.895125661  0.878133488  0.821581082
##  [36]  0.688640254  0.553917654 -0.061911711 -0.305962664 -0.380471001
##  [41] -0.694706979 -0.207917278 -1.265396352  2.168955965  1.207961998
##  [46] -1.123108583 -0.402884835 -0.466655354  0.779965118 -0.083369066
##  [51]  0.253318514 -0.028546755 -0.042870457  1.368602284 -0.225770986
##  [56]  1.516470604 -1.548752804  0.584613750  0.123854244  0.215941569
##  [61]  0.379639483 -0.502323453 -0.333207384 -1.018575383 -1.071791226
##  [66]  0.303528641  0.448209779  0.053004227  0.922267468  2.050084686
##  [71] -0.491031166 -2.309168876  1.005738524 -0.709200763 -0.688008616
##  [76]  1.025571370 -0.284773007 -1.220717712  0.181303480 -0.138891362
##  [81]  0.005764186  0.385280401 -0.370660032  0.644376549 -0.220486562
##  [86]  0.331781964  1.096839013  0.435181491 -0.325931586  1.148807618
##  [91]  0.993503856  0.548396960  0.238731735 -0.627906076  1.360652449
##  [96] -0.600259587  2.187332993  1.532610626 -0.235700359 -1.026420900

x[x < 0] <- NA
mean(x, na.rm = TRUE)

## [1] 0.7889055

##bai2
df <- data.frame(
  t = c(3, 5, 2, 7),
  y = c(2.5, 5.1, 2.8, 6.5)
)
df

##   t   y
## 1 3 2.5
## 2 5 5.1
## 3 2 2.8
## 4 7 6.5

sqrt(
  mean((df$t - df$y)^2)
)

## [1] 0.5361903