x-1 = 18 hoac y+5= 18

x=19 hoac y=13

(x-1).(y+5)=1.18=2.9

TH1{ x-1=1=> x=2

       {y+5=18 => y=13

TH2{x-1=18=> x=19

       {y+5=1 => y=-4

TH3{ x-1=2 => x=3

      {y+5=9 => y=4

TH4{x-1=9 => x=10

      {y+5=2 => y=-3

KL:.....

1+2-3-4+5+6-7-8+9+10-11-12+........+298-299-300+301+302 = 

1+2+(5-3)+(6-4)+(9-7)+(10-8)+…….+(301-299)+(302-300)=

Từ 302 đến 3 có số cặp là [(302-3):1+1]:2=150 cặp. Mà mỗi cặp có giá trị là 2

Vậy 1+2-3-4+5+6-7-8+9+10-11-12+........+298-299-300+301+302 = 

1+2+2×150=3+300=303

303 ok

\(2,\\ a,=2x^2+4x-3x-6-2x^2-4x-2=-3x-8\\ b,=\left[x-2+2\left(x+1\right)\right]^2=\left(x-2+2x+2\right)^2=9x^2\)

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

\(\dfrac{x}{3}=\dfrac{y}{5}=\dfrac{z}{7}=\dfrac{3x-2z}{3\cdot3-2\cdot7}=\dfrac{15}{-5}=-3\)

Do đó: x=-9; y=-15; z=-21

\(Y:\dfrac{7}{5}=\dfrac{5}{6}+\dfrac{5}{12}=\dfrac{15}{12}\\ Y=\dfrac{15}{12}\times\dfrac{7}{5}=\dfrac{7}{4}\)

\(y:\dfrac{7}{5}=\dfrac{5}{6}+\dfrac{5}{12}\\ \Leftrightarrow y:\dfrac{7}{5}=\dfrac{5\times2}{6\times2}+\dfrac{5}{12}\\ \Leftrightarrow y:\dfrac{7}{5}=\dfrac{10}{12}+\dfrac{5}{12}\\ \Leftrightarrow y:\dfrac{7}{5}=\dfrac{15}{12}\\ \Leftrightarrow y:\dfrac{7}{5}=\dfrac{5}{4}\\ \Leftrightarrow y=\dfrac{5}{4}\times\dfrac{7}{5}\\ \Leftrightarrow y=\dfrac{7}{4}\)

\(\Rightarrow C\)

y:7/5=5/6+5/12
y:7/5=5/4
y      =5/4x7/5
y      =7/4
đáp án:c

x=-6/19 (^-^)b