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\(\dfrac{28}{18};\dfrac{42}{27};\dfrac{56}{36};\dfrac{70}{45};\dfrac{84}{54};\dfrac{98}{63}\)

\(\left(x-3\right)\left(x+4\right)-2\left(3x-2\right)=\left(x-4\right)^2\\ \Leftrightarrow x^2+x-12-6x+4=x^2-8x+16\\ \Leftrightarrow x^2+x-12-6x+4-x^2+8x-16=0\\ \Leftrightarrow3x-24=0\\ \Leftrightarrow x=8\)

\(a,\dfrac{5}{13}\times\dfrac{4}{15}\times13=\dfrac{5\times4\times13}{13\times5\times3}=\dfrac{4}{3}\\ b,\left(\dfrac{3}{7}+\dfrac{5}{2}\right)\times\dfrac{7}{5}=\dfrac{3}{7}\times\dfrac{7}{5}+\dfrac{5}{2}\times\dfrac{7}{5}=\dfrac{3}{5}+\dfrac{7}{2}=\dfrac{6}{10}+\dfrac{35}{10}=\dfrac{41}{10}\\ c,\dfrac{1}{5}\times\dfrac{11}{18}+\dfrac{11}{18}\times\dfrac{3}{5}=\dfrac{11}{18}\times\left(\dfrac{1}{5}+\dfrac{3}{5}\right)=\dfrac{11}{18}\times\dfrac{4}{5}=\dfrac{22}{45}\)

123,8 - 34,15 - 12,49 - ( 5,85 - 2,49 ) + 10,2

=123,8 - 34,15 - 12,49 -  5,85 + 2,49  + 10,2

=(123,8+10,2)-(34,15+5,85)-(12,49-2,49)

=134-40-10

=84

Thay x=2, y=-1/2 vào B ta có:
\(B=x^3+2x^2y-4xy^2+2y-3\\=2^3+2.2^2.\left(-\dfrac{1}{2}\right)-4.2.\left(-\dfrac{1}{2}\right)^2+2.\left(-\dfrac{1}{2}\right)-3\\ =8-4-2-1-3\\ =-2\)

buff j :V

Thay x=1 vào A ta có:

\(A=3x^2-2x+1=3.1^2-2.1+1=3-2+1=2\)

\(x=\left|-2\right|\Rightarrow x=2\)

Thay x=2, y=-1, z=-1 vào A ta có:
\(A=2x-3y+4z^2\\ =2.2-3.\left(-1\right)+4.\left(-1\right)^2\\ =4+3+4.1\\ =4+3+4\\ =11\)