请大家指点，谢谢(20分)

已知两个实数a=1.2和b=3.4， 显示他们的 和 差 积 商。
源代码如下：
procedure TForm1.Button1click(Sender: Tobject);
const
a: real=1.2;
b: real=3.4;
var
s,D,M,V:AnsiString;
begin
Str(a+b:5:2,S);//将a+b结果取两位小数，总宽度5位转换为串
Str(a-b:5:2,D);
Str(a*b:6:3,M);
Str(a/b:6:3,V);
S :=‘和是：’+ S +。。。；//组织显示字符串
label1.caption := S;
end;
_----------------------------------------------------------
----------------------------------------------------------
我的问题是：
求他们a b 的和 差 积 商， 那在这里5：2 和6：3 是怎莫回事？
谢谢！

procedure Str(X [: Width [: Decimals ]]
var S);
5：2是5位宽度，小数2位。定义数据长度和小数位置而已。

是的，zywcd已经说的很清楚了：）

谢谢zywcd和Johnny_du，但我还是不明白，定义数据长度和小数位置是必须的过程吗？另外怎莫是5位宽度？

No,the length and the precision of the number X is not necessary, in other words, it's optional.

The procedure Str,formats a string and returns it to a variable.

procedure Str(X [: Width [: Decimals ]]
var S);

Description

Str converts X to a string representation according to the Width and Decimals formatting parameters. The effect is like a call to Write except the resulting string is stored in S instead of being written to a text file.

X is an integer-type or real-type expression. Width and Decimals are integer-type expressions. S is a string-type variable or a zero-based character array variable if extended syntax is enabled.

Here's an Example for this procedure:
function MakeItAString(I: Longint): string;

{ Convert any integer type to a string }
var
S: string[11];
begin
Str(I, S);
Result:= S;
end;

begin
Canvas.TextOut(10, 10, MakeItAString(-5322));
end;
So when you call this function in an application, it'll draw some Numbers in the window with the given formats.