var k1=1,k2=1,find1;
     if(a==c || b==d){k1=1;k2=-1}
else if(gcd(a,c)>1){k1=c/gcd(a,c);k2=-a/gcd(a,c)}
else if(gcd(b,d)>1){k1=d/gcd(b,d);k2=-b/gcd(b,d)}
else if(a==1){k1=c;k2=-1}
else if(b==1){k1=d;k2=-1}
else if(c==1){k1=1;k2=-a}
else if(d==1){k1=1;k2=-b}
else {k1=c;k2=-a};
if(k1==-1){k1=1;k2=-k2;};
if(k2==-1){k1=-k1;k2=1};
find1=(k1*a+k2*c==0?1:0);
document.writeln("First eliminate one variable.<br>",
(k1!=1?"Multiply the top equation by "+showterms([[k1,1]]):""),
(k2!=1?(k1!=1?"; m":"M")+"ultiply the bottom equation by "+showterms([[k2,1]]):""),
   (k1!=1||k2!=1?":<br>":""),
   "<blockquote>",
    showterms([[k1*a,v[0]],[k1*b,v[1]]," = ",[k1*(a*x+b*y),1]]),"<br>",
    showterms([[k2*c,v[0]],[k2*d,v[1]]," = ",[k2*(c*x+d*y),1]]),"</blockquote>",
"Add these equations:<blockquote>",
    (find1==0&&k1*a+k2*c==1 ||find1==1&&k1*b+k2*d==1
      ?"<span class=u>&nbsp;":""),
    showterms([[k1*a+k2*c,v[0]],[k1*b+k2*d,v[1]]," = ",[k1*(a*x+b*y)+k2*(c*x+d*y),1]]),
    (find1==0&&k1*a+k2*c==1||find1==1&&k1*b+k2*d==1?"&nbsp;</span>":""),
    
    (find1==0?( k1*a+k2*c!=1?"<br>"+iffch+"<span class=u>&nbsp;"+showterms([[1,v[0]]," = ",[x,1]])+"&nbsp;</span>":"")
             :( k1*b+k2*d!=1?"<br>"+iffch+"<span class=u>&nbsp;"+showterms([[1,v[1]]," = ",[y,1]])+"&nbsp;</span>":"")),
 "</blockquote>Substitute this value in the first (original) equation:<blockquote>",
    (find1==0&&a*x!=0 || find1==1&& b*y!=0
     ?showterms([[(find1==0?a*x:a),find1==0?1:v[0]],
               [(find1==1?b*y:b),find1==1?1:v[1]]," = ",[a*x+b*y,1]])+"<br>"+iffch
     :""),
    (find1==0&&b==1||find1==1&&a==1?"<span class=u>&nbsp;":""),
    (find1==0?showterms([[b,v[1]]," = ",[b*y,1]]):showterms([[a,v[0]]," = ",[a*x,1]])),           
    (find1==0&&b==1||find1==1&&a==1?"&nbsp;</span>":""),"<br>",
    (find1==0&&b!=1||find1==1&&a!=1
    ?iffch+"<span class=u>&nbsp;"+showterms([[1,v[1-find1]]," = ",[find1==0?y:x,1]])+"&nbsp;</span>"
    :""),
    "</blockquote><br>"
    
);