var liczbaProd=8;

function qwe(id) {
 return document.getElementById(id);
}

function podlicz() {
 var x=0;
 for(i=1;i<=liczbaProd;i++) {
  if(qwe(i+'a').checked) {
   x+=qwe(i+'b').value*qwe(i+'c').value;
  }
 }
 qwe('x').value=x;
}

function a_plus_b(form) {
a=eval(form.a.value)
b=eval(form.b.value)
c=a+b
form.netto.value = c
}
function a_minus_b(form) {
a=eval(form.a.value)
b=eval(form.b.value)
c=a-b
form.kwota.value=c
}
function a_times_b(form) {
a=eval(form.a.value)
b=eval(form.b.value)
c=a*b
form.kwota.value=c
}
function a_div_b(form) {
a=eval(form.a.value)
b=eval(form.b.value)
c=a/b
form.kwota.value = c
}
function a_pow_b(form) {
a=eval(form.a.value)
b=eval(form.b.value)
c=Math.pow(a, b)
form.kwota.value = c
}