ta có:
3/5C=3/18.21+3/21.24+3/24.27+.....+3/123.126
3/5C=1/18-1/21+1/21-1/24+1/24-1/27+.....+1/123-1/126
3/5C=1/18-1/126
3/5C=7/126-1/126
3/5C=1/21
C=1/21:3/5=5/63
Bài 15
a)-3/7+15/26-(2/13-3/7)
=-3/7+15/26-2/13+3/7=(-3/7+3/7)+(15/26-2/13)
=0+(15/26-4/26)=0+11/26=11/26
d)(377/-231-123/89+34/791).(1/6-1/8-1/24)
=(377/-231-123/89+34/791).(4/24-3/24-1/24)
=(377/-231-123/89+34/791).0
=0
Bài 16:
a)2/3x-3/2x=5/12
(2/3-3/2).x=5/12
(4/6-9/6).x=5/12
-5/6.x =5/12
x =5/12:(-5/6)
x = -1
