90a,bc : a,bc = 721 

(900 + a,bc) : a,bc = 721

900 : a,bc + a,bc  : a,bc = 721

900 : a,bc + 1 = 721

900 : a,bc = 721 - 1 = 720

a,bc = 900 : 720 = 1,25

Vậy a = 1; b = 2; c = 5

=> (900 + a,bc . 0,01) : (a,bc . 0,01) = 721

=> 900 :  (a,bc . 0,01) + (a,bc . 0.01) :  (a,bc . 0,01) = 721

=> 900 : (a,bc . 0,01) + 1 = 721

=> 900 :  (a,bc . 0,01) = 720

=> a,bc . 0,01 = 1,25

=> abc = 125

Vậy a = 1 ; b = 2 và c = 5

k mik vs

mik k lại cho

k mik vs

mik k lại cho

D

B

B

a: \(\dfrac{877-160a}{1000-90a}=\dfrac{35.5}{100}\)

\(\Leftrightarrow87700-16000a=35500-3195a\)

\(\Leftrightarrow-12805a=-52200\)

hay a=10440/2561

b: \(\dfrac{642-120a}{1000-108a}=44.5\)

\(\Leftrightarrow642-120a=44500-4806a\)

=>4686a=43858

hay a=21929/2343

a, x = \(\dfrac{-210}{-35}=6\)

b, x = \(\dfrac{42}{-7}=-6\)

c, x = \(\dfrac{180}{-12}=-15\)

d, x² = 9

=> x = 3 hoặc x = -3

(-35).x=-210

x         =210:35

x         =6

3^2=9

\(B=\frac{a}{x\left(x+a\right)}+\frac{a}{\left(x+a\right)\left(x+2a\right)}+\frac{a}{\left(x+2a\right)\left(x+3a\right)}+....+\frac{a}{\left(x+9a\right)\left(x+10a\right)}+\frac{1}{x+10a}\)

  \(=\frac{1}{x}-\frac{1}{x+a}+\frac{1}{x+a}-\frac{1}{x+2a}+\frac{1}{x+2a}-\frac{1}{x+3a}+....+\frac{1}{x+9a}-\frac{1}{x+10a}+\frac{1}{x+10a}\)

\(=\frac{1}{x}\)