giúp với k cho ai nhanh và đúng nhất 

dễ mik dùng cácg rút nha!! ta rút b
 => Có biểu thức (2 x b : 3) + b + (5 x b : 4) = 21
  nhấn máy tính CASIO => b =7,2
     a = (2 x b :3) = (2 x 7,2 :3) =4,8
     c= (5 x b :4) = (5 x 7,2 :4)=9
=> 3a-b+c=3 x 4,8 - 7,2 + 9=16,2
   Bn k cho mik nha!!!

\(\frac{a}{18}=\frac{4}{3}\Rightarrow a.3=4.18\Rightarrow a=\frac{4.18}{3}=4.6=24\)

\(\frac{20}{b}=\frac{4}{3}\Rightarrow b.4=20.3\Rightarrow b=\frac{20.3}{4}=5.3=15\)

\(\frac{c}{21}=\frac{4}{3}\Rightarrow c.3=21.4\Rightarrow c=\frac{21.4}{3}=7.4=28\)

Vậy a = 24;b = 15; c = 28.

\(\frac{a}{18}=\frac{4}{3}\Rightarrow3.a=4.18\Rightarrow a=24\)

\(\frac{24}{18}=\frac{20}{b}\Rightarrow24.b=20.18\Rightarrow b=15\)

\(\frac{20}{15}=\frac{c}{21}\Rightarrow15.c=20.21\Rightarrow c=28\)

Vậy a=24; b=15;b=28

khó quá thông cảm vì mình mới lớp 5

a)\(\left(x-\frac{1}{2}\right)^{2016},\left|\frac{3}{4}-y\right|\ge0\)

\(\left(x-\frac{1}{2}\right)^{2016}+\left|\frac{3}{4}-y\right|=0\)

\(\Rightarrow\orbr{\begin{cases}\left(x-\frac{1}{2}\right)^{2016}=0\\\left|\frac{3}{4}-y\right|=0\end{cases}}\)

\(\Rightarrow\orbr{\begin{cases}x-\frac{1}{2}=0\\\frac{3}{4}-y=0\end{cases}}\)

\(\Rightarrow\orbr{\begin{cases}x=\frac{1}{2}\\y=\frac{3}{4}\end{cases}}\)

b)\(\frac{a}{b+c}=\frac{b}{a+c}=\frac{c}{a+b}\)

\(\Rightarrow\frac{b+c}{a}=\frac{a+c}{b}=\frac{a+b}{c}\)

\(\Rightarrow\frac{b+c}{a}-\frac{a+c}{b}-\frac{a+b}{c}=0\)

Gọi \(M=\frac{a-b}{c}+\frac{b-c}{a}+\frac{c-a}{b},\)ta có :

\(M.\frac{c}{a-b}=1+\frac{c}{a-b}\left(\frac{b-c}{a}+\frac{c-a}{b}\right)=1+\frac{c}{a-b}.\frac{b^2-bc+ac-a^2}{ab}\)

\(=1+\frac{c}{a-b}.\frac{\left(a-b\right)\left(c-a-b\right)}{ab}=1+\frac{2c^2}{ab}=1+\frac{2c^3}{abc}\)

Tương tự : \(M.\frac{a}{b-c}=1+\frac{2a^3}{abc},M.\frac{b}{c-a}=1+\frac{2b^3}{abc}.\)

Vậy \(A=3+\frac{2\left(a^3+b^3+c^3\right)}{abc}=9\)

mik ko hieu lam

\(a\left(a+b+c\right)=-12\)

\(b\left(a+b+c\right)=18\)

\(c\left(a+b+c\right)=30\)

\(a\left(a+b+c\right)+b\left(a+b+c\right)+c\left(a+b+c\right)=-12+18+30\)

\(\left(a+b+c\right)\left(a+b+c\right)=36\)

\(\left(a+b+c\right)^2=\left(\pm6\right)^2\)

\(a+b+c=\pm6\)

Th1:

\(a+b+c=6\)

\(\left[\begin{array}{nghiempt}a\times6=-12\\b\times6=18\\c\times6=30\end{array}\right.\)

\(\left[\begin{array}{nghiempt}a=-\frac{12}{6}\\b=\frac{18}{6}\\c=\frac{30}{6}\end{array}\right.\)

\(\left[\begin{array}{nghiempt}a=-2\\b=3\\c=5\end{array}\right.\)

Th2:

\(a+b+c=-6\)

\(\left[\begin{array}{nghiempt}a\times\left(-6\right)=-12\\b\times\left(-6\right)=18\\c\times\left(-6\right)=30\end{array}\right.\)

\(\left[\begin{array}{nghiempt}a=\frac{-12}{-6}\\b=\frac{18}{-6}\\c=\frac{30}{-6}\end{array}\right.\)

\(\left[\begin{array}{nghiempt}a=2\\b=-3\\c=-5\end{array}\right.\)

bạn dùng TC dãy tỉ số bằng nhau đi

cộng vào là ra kết quả ngay mà

Ta có: \(\frac{a}{2}=\frac{b}{3};\frac{b}{4}=\frac{c}{5}\Rightarrow\frac{a}{8}=\frac{b}{12}=\frac{c}{15}\)

\(\Rightarrow\frac{a}{8}=\frac{b}{12}=\frac{c}{15}=\frac{a+b+c}{8+12+15}=\frac{21}{35}=\frac{3}{5}\)

\(a=\frac{3}{5}.8=\frac{24}{5}\)

\(b=\frac{3}{5}.12=\frac{36}{5}\)

\(c=\frac{3}{5}.15=9\)

\(\Rightarrow3a-b+c=3.\frac{24}{5}-\frac{36}{5}+9=\frac{81}{5}\)

Vậy 3a - b + c = 81/5

\(\frac{a}{2}=\frac{b}{3};\frac{b}{4}=\frac{c}{5}\)

=> \(\frac{a}{8}=\frac{b}{12};\frac{b}{12}=\frac{c}{15}\)

=>\(\frac{a}{8}=\frac{b}{12}=\frac{c}{15}\)và a + b + c =21

Áp dụng tính chất của dãy tỉ số bằng nhau ta có :

\(\frac{a}{8}=\frac{b}{12}=\frac{c}{15}=\frac{a+b+c}{8+12+15}=\frac{21}{35}=\frac{3}{5}\)

=> a = \(\frac{24}{5}\)

     b = \(\frac{36}{5}\)

     c = 9

=> 3a - b + c = 16 , 2

Vậy 3a - b + c = 16 , 2

\(\frac{a}{b}=\frac{-3}{5};a+b=4\)