4.Tinh chat co ban cua phan so la 
+Neu ta nhan ca tu va mau cua mot phan so voi cung mot so nguyen khac 0 thi ta duoc mot phan so bang phan so da cho.
a/b=a.m/b.m voi m thuoc Z va m khac 0.
+Neu ta chia ca tu va mau cua mot phan so cho cung mot uoc chung cua chung thi ta duoc mot phan so bang phan so da cho.
a/b=a:n/b:n voi n thuoc UC(a,b)
5.
+Rut gon phan so la lam cho phan so do tro nen toi gian.
+Phan so toi gian la phan so ma tu va mau chi co uoc chung la 1 va -1
VD:3/9=1/3;5/6;12/24=1/2;...
6.
Quy tac:
Buoc 1:Tim mot boi chung cua cac mau(thuong la BCNN)de lam mau chung.
Buoc 2:Tim thua so phu cua moi mau(bang cach chia mau chung cho tung mau)
Buoc 3:Nhan tu va mau cua moi phan si voi thua so phu tuong ung.

7.Muon so sanh hai phan so ta nen:
 *Cung mau:
-Trong hai phan so co cung mot mau duong,phan so nao co tu lon hon thi lon hon.
*Khac mau:
-Ta viet chung duoi dang hai phan so co cung mot mau duong roi so sanh cac tu voi nhau:Phan so nao co tu lon hon thi lon hon.
8.Quy tac cong phan so:
*Cung mau:
-Ta cong cac tu va giu nguyen mau.
*Khac mau:
-Ta viet chung duoi dang hai phan so co cung mot mau roi cong cac tu va giu nguyen mau chung.
9.
Co tinh chat:giao hoan,ket hop,cong voi so 0
VD:giao hoan:1/4+2/4=2/4+1/4
       ket hop:(2/3+4/5)+1/3=4/5+(2/3+1/3)
        cong voi so 0 la:1/3+0=0+1/3=1/3

10.
Hai so doi nhau neu tong cua chung bang 0.
Hai so nghich dao neu tich cua chung bang 1,
VD:So doi nhau:-1/4;1/4
       So nghich dao:-3;1/3
11
Nhan phan so:Tu nhan tu,mau nhan mau
 Chia phan so:Nhan so bi chia voi so nghich dao cua so chia.
12.
Phep nhan phan so co tinh chat:Giao hoan,ket hop,nhan voi so 1,tinh chat phan phoi cua phep nhan doi voi phep cong:
Giao hoan:2/3. 1/5= 1/5 . 2/3
Ket hop:(1/5 . 2/5).4/2=(1/5 . 4/2). 2/5
Nhan voi so 1:2/3.1=1.2/3=2/3
Tinh chat phan phoi:1/5.(2/4. 2/20)=(1/5.2/20).2/4