I have a vector `a`

```
a = [86 100 41 93 75 61 76 92 88 97]

```

And I want to calculate the `std` and `mean` by 
myself:

```
>> mean(a)

ans =

   80.9000

>> std(a)^2

ans =

  335.2111

```

But when I do it like that I get wrong variance:

```
>> avg = mean(a)

avg =

   80.9000

>> var = sum(a.^2)/length(a) - avg^2

var =

  301.6900

```

What do I miss here ?

why `sum(a.^2)/length(a) - avg^2 != std(a)^2` ?