dimension a(15)
   d(i)=a(i)*0.049
这时的a(i)是未定义的，也就不确定了

找到问题了，已经解决，非常感谢！
dimension a(15),c(15),d(15)
m=val(thisform.text1.value)
n=val(thisform.text2.value)
b=VAL(thisform.text3.value)
s=0
a(1)=m+n
for i=1to 14
   d(i)=a(i)*0.049
   c(i)=b-d(i)
   a(i+1)=a(i)-c(i)
   s=s+d(i)   
endfor
d(15)=a(15)*0.049
p=(s+d(15))/15
thisform.text4.value=p