Câu 4:
a.x:14/7=1/3
⇒x = 1/3 × 14/7
⇒x=2/3
Vậy,...
b,x×3/5-13/10=-1/3
⇒x×3/5=-1/3+-13/10
⇒x×3/5=49/30
⇒x=49/30:3/5
⇒x=49/18
Vậy,...
c,1/9×x+2/3×x=1/2
⇒2x(1/9+2/3)=1/2
⇒2x × 7/9=1/2
⇒2x=1/2:7/9
⇒2x=9/14
⇒x=9/14:2
⇒x=9/28
Vậy,...
Bài 6:
ta có:
1/1.2+1/2.3+...+1/49.50
=1/1-1/2+1/3-1/4+...+1/49-1/50
=1/1-1/5=49/50
vì 49/50>1 nên 1/1.2+1/2.3+...+1/49.50>1
vậy,...