To em co so ban la 

5+5=10 ban

dap so 10 ban 

a) 0,8 % = C: \(\frac{8}{1000}\)

b)C: 100

nha mình sẽ tích lại cho

BÀi giải

A= 1/1 x 2+ 1/2 x 3+1/3 x 4 +........+ 1/2014 x 2015

A= 1/1 - 1/2 + 1/2 - 1/3 +1/3 - 1/4 + 1/4 -........+ 1/2014 - 1/2015

A= 1/1 - 1/2015

A=2015/2015 - 1/2015

A= 2014/2015

vtb=\(\frac{s1}{t1}+\frac{s2}{t2}=\frac{\frac{2}{3}t.v1+}{\frac{2}{3}t}\frac{\frac{1}{3}t.v2}{\frac{1}{3}t}=\frac{\frac{2}{3}.}{\frac{2}{3}}\frac{\frac{1}{3}\left(t.v1+t.v2\right)}{\frac{1}{3}\left(t+t\right)}=\frac{\frac{2}{9}\left(t.v1+t.v2\right)}{\frac{2}{9}\left(t+t\right)}=\frac{t.v1+t.v2}{2t}=\frac{v1+v2}{2}\)