[CAS] inconsistent evaluation

[compsystems]

Hello Hp-Prime user and developers

Flags: Simplify none, exact mode off, and cas mode on

run line by line the following script

Quote:c:=1+(-a-b); [enter] returns 1.-a-b
but
c1:=1-a-b; [enter] returns 1.-a-b

c==c1 [enter] returns true

x1:=1/10.;
y1:=1/9.;
x2:=100/8.;
y2:=100/3.;
x3:=100/4.;
y3:=100/9.;
x:=379/70.;
y:=904/63.;
d:=(y2-y3)*(x1-x3)+(x3-x2)*(y1-y3);
na:=(y2-y3)*(x-x3)+(x3-x2)*(y-y3);
nb:=(y3-y1)*(x-x3)+(x1-x3)*(y-y3);
a:=na/d;
b:=nb/d; [enter] ....

c:=1+(-a-b); [enter] returns 0
but
c1:=1-a-b; [enter] returns −1.7763568394*10^−15

Why when evaluate c <> c1? if c is equal to c1

[compsystems]

aslo I can not run the following program, I can not find error

PHP Code:

#cas
testP():=
    begin
        local x1,y1,x2,y2,x3,y3;
        local x,y,d;
        local na,nb,a,b,c,c1;
        x1:=1/10.;
        y1:=1/9.;
        x2:=100/8.;
        y2:=100/3.;
        x3:=100/4.;
        y3:=100/9.;
        x:=379/70.;
        y:=904/63.;
        d:=(y2-y3)*(x1-x3)+(x3-x2)*(y1-y3);
        na:=(y2-y3)*(x-x3)+(x3-x2)*(y-y3);
        nb:=(y3-y1)*(x-x3)+(x1-x3)*(y-y3);
        a:=na/d;
        b:=nb/d;
        c:=1+(-a-b);
        c1:=1-a-b;
        return({ c, c1 });
    end;
#end 


Who can run the previous program?