b)

X : 5 = 135 ( dư 3 )

X = 135 x 5 

X = 675

X = 675 + 3

X = 678

b) 5 x X = 4680

X = 4680 : 5

X = 930

c) ( x : 7 ) x 3 = 162

X x 3 = 162 x 7

X x 3 = 1134

X = 1134 : 3

X = 378

Sửa đề \(D=\frac{a^3+3^3}{b^3+4^3}\)biết \(\frac{a+3}{a-3}=\frac{b+4}{b-4}\)

\(\Leftrightarrow\left(a+3\right)\left(b-4\right)=\left(a-3\right)\left(b+4\right)\)

\(\Leftrightarrow ab-4a+3b-12=ab+4a-3b-12\)

\(\Leftrightarrow8a=6b\)

\(\Leftrightarrow\frac{a}{6}=\frac{b}{8}\Leftrightarrow\frac{a}{3}=\frac{b}{4}\)

Đặt \(\frac{a}{3}=\frac{b}{4}=k\)\(\Rightarrow a=3k,b=4k\)

\(\Rightarrow D=\frac{a^3+3^3}{b^3+4^3}=\frac{\left(3k\right)^3+3^3}{\left(4k\right)^3+4^3}\)

\(=\frac{3^3\left(k^3+1\right)}{4^3\left(k^3+1\right)}=\frac{3^3}{4^3}=\frac{27}{64}\)

TL: 
8 nhé 

HNJK

\(P=\dfrac{3x^2+1-3x^2+6x-3}{3x^2+1}=1-\dfrac{3\left(x-1\right)^2}{3x^2+1}\le1\)

\(P_{max}=1\) khi \(x=1\)

w

360:4/9=160

160 : 2/5 = 64  vay 360-160-64 =136

\(\left(3x-1\right).\left(\frac{-1}{2}x+5\right)=0.\)

\(TH1:3x-1=0\)

\(3x=0+1\)

\(3x=1\)

\(x=\frac{1}{3}\)

\(TH2:\frac{-1}{2}x+5=0\)

\(\frac{-1}{2}x=-5\)

\(x=-5\div\frac{-1}{2}\)

\(x=10\)

\(\left(3x-1\right).\left(\frac{-1}{2}x+5\right)=0.\)

TH1: 

\(3x-1=0\)

\(3x=0+1\)

\(3x=1\)

\(x=1\div3\)

\(x=\frac{1}{3}\)

TH2: 

\(\frac{-1}{2}x+5=0\)

\(\frac{-1}{2}x=0-5\)

\(\frac{-1}{2}x=-5\)

\(x=-5\div\frac{-1}{2}\)

\(\Leftrightarrow x=-5.\left(-2\right)\)

\(x=10\)

\(\Rightarrow\orbr{\begin{cases}x=\frac{1}{3}\\x=10\end{cases}}\)