one.jsp
```java
%@ page contentType="text/html" %>
<%@ page pageEncoding="utf-8" %>
<%@ taglib tagdir="/WEB-INF/tags" prefix="getTriangleArea" %>
<HTML><body bgcolor = cyan>
<p style="font-family:宋体;font-size:36;color:blue">
<%-- 使用Tag标记： --%>
<getTriangleArea:GetArea sideA="15" sideB="16" sideC="20"/>
<%
double result=Double.parseDouble(area);
String S1=String.format("%.3f",result);
out.print("传递回来的三角形面积（小数点保留3位）为："+S1);
%>
</p>
</body></HTML>
two.jsp
```java
<%@ page contentType="text/html" %>
<%@ page pageEncoding="utf-8" %>
<%@ taglib tagdir="/WEB-INF/tags" prefix="getTriangleArea" %>
<HTML><body bgcolor = cyan >
<p style="font-family:宋体;font-size:36;color:blue">
<%-- 使用Tag标记： --%>
<getTriangleArea:GetArea sideA="15" sideB="16" sideC="20"/>
<%
double result =area.doubleValue();
String S2=String.format("%.6f",result);
out.print("传递回来的三角形面积（小数点保留6位）为："+S2);
%>
</p>
</body></HTML>
GetArea.tag
<%@ tag pageEncoding="utf-8" %>
<%@ attribute name="sideA" required="true" %>
<%@ attribute name="sideB" required="true" %>
<%@ attribute name="sideC" required="true" %>
<%@ variable name-given= "area" variable-class= "java.lang.Double"
scope= "AТ_ END" %>
<%
double a=Double.parseDouble(sideA);
double b=Double.parseDouble(sideB);
double c=Double.parseDouble(sideC);
if(a+b>c&&a+c>b&&c+b>a){
double p=(a+b+c)/2.0;
double result=Math.sqrt(p*(p-a)*(p-b)*(p-c)) ;
jspContext.setAttribute("area" ,new Double(result));
}
else{
jspContext.setAttribute("area" ,new Double(-1));
}
%>
用浏览器访问总是报错，请问是哪里出了问题？
