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a2 = b2 + c2 = 2c2 - 8 +c2 = 3c2 - 8
=> M = 5.( 3c2 - 8 ) - 7.( 2c2 - 8) - c2 = 15 c2 - 40 - 14 c2 + 56 - c2 = (15 c2 -14c2 - c2) -40 + 56 = 16
![](https://rs.olm.vn/images/avt/0.png?1311)
a: \(\Leftrightarrow x\in\left\{1;-1;2;-2;3;-3;4;-4;6;-6;9;-9;12;-12;18;-18;36;-36\right\}\)
mà -3<x<30
nên \(x\in\left\{-2;-1;1;2;3;4;6;9;12;18\right\}\)
b: \(\Leftrightarrow x\in\left\{0;4;-4;8;-8;12;-12;...\right\}\)
mà -16<=x<20
nên \(x\in\left\{-16;-12;-8;-4;0;4;8;12;16\right\}\)
c: \(\Leftrightarrow x-1+4⋮x-1\)
\(\Leftrightarrow x-1\in\left\{1;-1;2;-2;4;-4\right\}\)
hay \(x\in\left\{2;0;3;-1;5;-3\right\}\)
d: \(\Leftrightarrow2x+4-5⋮x+2\)
\(\Leftrightarrow x+2\in\left\{1;-1;5;-5\right\}\)
hay \(x\in\left\{-1;-3;3;-7\right\}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
a)=-7/21+8/24
=-1/3+1/3
=0
b)=-3/5.(2/7+5/7)+23/5
=-3/5.7/7+23/5
=-3/5.1+23/5
=-3/5+23/5
=20/5=4
c)=75/100-11/2+5/10:5/12+1/4
=3/4-11/2+1/2:5/12+1/4
=3/4+-11/2+1/2.12/5+1/4
=3/4+-22/4+6/5+1/4
=-19/4+6/5+1/4
=(-19/4+1/4)+6/5
=-18/4+6/5
=-9/2+6/5
=-45/10+12/10
=-23/10
![](https://rs.olm.vn/images/avt/0.png?1311)
\(\frac{a}{b}=\frac{c}{d}\)=\(\frac{a}{c}=\frac{b}{d}\)=>\(\frac{a^2}{c^2}=\frac{b^2}{d^2}=\frac{a^2+b^2}{c^2+d^2}\)(2)
=>\(\frac{a^2}{c^2}=\frac{b^2}{d^2}=\frac{a^2-b^2}{c^2-d^2}\)(3)
=>\(\frac{a+b}{c+d}=\frac{\left(a+b\right)^2}{\left(c+d\right)^2}\)(4)
=>Từ (1),(2),(3),(4)=>\(\frac{a}{b}=\frac{a^2-b^2}{c^2-d^2}=\frac{a^2+b^2}{c^2+d^2}=\frac{\left(a+b\right)^2}{\left(c+d\right)^2}\)(đpcm)
![](https://rs.olm.vn/images/avt/0.png?1311)
A=1+2+3+...+2018=(1+2018)+(2+2017)+...(1009+1010)=2019x1009=2037171
B=(1+2019)+(3+2017)+...+(1009+1011)=2020x505=1020100
C=(2020+2)+(2018+4)+...+(1010+1012)=2022x505=1021110