AB=AC+CB=3CB+CB=4CB

=>4CB=40

=>CB=10

=>AC=40-10=30

b

C là trung điểm của BM =>CM=CB =>BM=2CB=>BM=2*10=20

=>AM=40-20=20

A = \(\dfrac{1}{20}\) + \(\dfrac{1}{30}\) + \(\dfrac{1}{42}\) + \(\dfrac{1}{56}\) + \(\dfrac{1}{72}\) + \(\dfrac{1}{90}\) + \(\dfrac{1}{110}\) + \(\dfrac{1}{132}\)

A = \(\dfrac{1}{4\times5}\) + \(\dfrac{1}{5\times6}\) + \(\dfrac{1}{6\times7}\)+ \(\dfrac{1}{7\times8}\)+\(\dfrac{1}{8\times9}\)+ \(\dfrac{1}{9\times10}\) + \(\dfrac{1}{10\times11}\)+\(\dfrac{1}{11\times12}\)

A = \(\dfrac{1}{4}\)-\(\dfrac{1}{5}\) +\(\dfrac{1}{5}\)-\(\dfrac{1}{6}\) +.....+\(\dfrac{1}{11}\) - \(\dfrac{1}{12}\)

A = \(\dfrac{1}{4}\) - \(\dfrac{1}{12}\)

A = \(\dfrac{1}{6}\)

Là số nào gấp 9 lần, số nào giảm 6 lần

`1/10+2/20+3/30+4/40+5/50+6/60+7/70+8/80+9/90`

`=1/10+1/10+1/10+1/10+1/10+1/10+1/10+1/10+1/10`

`=1/10xx9`

`=9/10`

giúp mình bài này nha

2,8 x 92 + 5,6 : 0,25

= 2,8 x 92 + 2,8 x 2 x 4

= 2,8 x 92 + 2,8 x 8

= 2,8 x (92+8)

= 2,8 x 100

= 280

Lời giải:
$A=\frac{1}{2^2}+(\frac{1}{3^2}+\frac{1}{4^2}+...+\frac{1}{2014^2})$

$< \frac{1}{2^2}+(\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{2013.2014})$

$=\frac{1}{4}+(\frac{3-2}{2.3}+\frac{4-3}{3.4}+...+\frac{2014-2013}{2013.2014})$

$=\frac{1}{4}+(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{2013}-\frac{1}{2014})$

$=\frac{1}{4}+(\frac{1}{2}-\frac{1}{2014})$

$< \frac{1}{4}+\frac{1}{2}=\frac{3}{4}$ (đpcm)

Sos

Đkxđ:

y≥0

x-1≠0 => x≠1

số bé = 13,6:(3-2) x 2=27,2