Tính nhanh

8A.Tính nhanh:

1)A = 1 + 2 + 3 +.....................+ 50

2)B = 2+4+6+8+........................+100

3)C = 1+3+5+7+........................+99

4)D= 2+5+8+11+........................+98

5)A = 1+2+3+4+..........................+25

6)B = 2+4+6+8+..........................+50

7)C = 3+5+7+9+..........................+51

8)D = 1+5+9+13+...........................+81
mik cần tính nhanh ko cần thường.Thanks!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!