a: \(\dfrac{4}{15}-\left(2,9-\dfrac{11}{15}\right)\)

\(=\dfrac{4}{15}-2,9+\dfrac{11}{15}\)

=1-2,9=-1,9

b: \(\left(-36,75\right)+\left(\dfrac{37}{10}-63,25\right)-\left(-6,3\right)\)

\(=-36,75-63,25+\dfrac{37}{10}+6,3\)

=-100+10

=-90

c: \(6,5-\left(-\dfrac{10}{71}\right)-\left(-\dfrac{7}{2}\right)-\dfrac{7}{17}\)

\(=6,5+\dfrac{10}{71}+3,5-\dfrac{7}{17}\)

\(=10+\dfrac{10}{71}-\dfrac{7}{17}=\dfrac{11743}{1207}\)

d: \(\left(-39,1\right)\cdot\dfrac{13}{25}-60,9\cdot\dfrac{13}{25}\)

\(=\dfrac{13}{25}\left(-39,1-60,9\right)\)

\(=\dfrac{13}{25}\cdot\left(-100\right)=-52\)

a: \(8m^2=\dfrac{8}{1000000}km^2=\dfrac{1}{125000}km^2=0,000008km^2\)

\(8cm^2=\dfrac{8}{10000}m^2=\dfrac{1}{1250}m^2=0,0008m^2\)

\(4mm^2=\dfrac{4}{1000000}m^2=\dfrac{1}{250000}m^2=0,000004m^2\)

b: \(7dm^2=\dfrac{7}{100}m^2=0,07m^2\)

\(2m^2=\dfrac{1}{50}dam^2=0,02m^2\)

\(5cm^2=\dfrac{5}{10000}m^2=\dfrac{1}{2000}m^2=0,0005m^2\)

a: \(BM=\dfrac{1}{4}BC\)

\(BN=\dfrac{1}{2}BC\)(N là trung điểm của BC)

Do đó: BN=2BM

=>M là trung điểm của BN

=>MB=MN

Xét ΔMBE và ΔMNA có

MB=MN

\(\widehat{BME}=\widehat{NMA}\)(hai góc đối đỉnh)

ME=MA

Do đó: ΔMBE=ΔMNA

=>\(\widehat{MBE}=\widehat{MNA}\)

=>BE//NA

Xét ΔMAB và ΔMEN có

MA=ME

\(\widehat{AMB}=\widehat{EMN}\)(hai góc đối đỉnh)

MB=MN

Do đó: ΔMAB=ΔMEN

=>AB=EN

1

\(A=\left(\dfrac{x+1}{x^3-1}-\dfrac{1}{x-1}\right)\left(\dfrac{x+2}{x-1}-\dfrac{1}{x}\right)\left(x\ne1;0\right)\\ =\left[\dfrac{x+1}{\left(x-1\right)\left(x^2+x+1\right)}-\dfrac{x^2+x+1}{\left(x-1\right)\left(x^2+x+1\right)}\right]\left[\dfrac{x\left(x+2\right)}{x\left(x-1\right)}-\dfrac{x-1}{x\left(x-1\right)}\right]\\ =\dfrac{x+1-x^2-x-1}{\left(x-1\right)\left(x^2+x+1\right)}\cdot\dfrac{x^2+2x-x+1}{x\left(x-1\right)}\\ =\dfrac{-x^2}{\left(x-1\right)\left(x^2+x+1\right)}\cdot\dfrac{x^2+x+1}{x\left(x-1\right)}\\ =\dfrac{-x}{\left(x-1\right)^2}\\ =\dfrac{-x}{x^2-2x+1}\)

ĐKXĐ: \(x\notin\left\{1;0\right\}\)

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

\(=\left(\dfrac{x+1}{\left(x-1\right)\left(x^2+x+1\right)}-\dfrac{1}{x-1}\right)\cdot\left(\dfrac{x\left(x+2\right)-x+1}{x\left(x-1\right)}\right)\)

\(=\dfrac{x+1-x^2-x-1}{\left(x-1\right)\left(x^2+x+1\right)}\cdot\dfrac{x^2+x+1}{x\left(x-1\right)}\)

\(=\dfrac{-x^2}{\left(x-1\right)\cdot x\left(x-1\right)}=\dfrac{-x}{\left(x-1\right)^2}\)

$2^{4-x}=128$

$\Rightarrow 2^{4-x}=2^7$

$\Rightarrow 4-x=7$

$\Rightarrow x=4-7$

$\Rightarrow x=-3$

\(2^{4-x}=128\)

\(2^{4-x}=2^7\)

\(4-x=7\)

      \(x=4-7\)

      \(x=-3\)

a: Xét (O) có

CM,CA là các tiếp tuyến

Do đó: CM=CA và OC là phân giác của góc MOA

Xét (O) có

DM,DB là các tiếp tuyến

Do đó: DM=DB và OD là phân giác của góc MOB

AC+BD

=CM+MD

=CD
b: \(\widehat{COD}=\widehat{COM}+\widehat{DOM}=\dfrac{1}{2}\cdot\widehat{MOA}+\dfrac{1}{2}\cdot\widehat{MOB}\)

\(=\dfrac{1}{2}\left(\widehat{MOA}+\widehat{MOB}\right)=\dfrac{1}{2}\cdot\widehat{AOB}=90^0\)

=>ΔCOD vuông tại O

c: Xét ΔCOD vuông tại O có OM là đường cao

nên \(OM^2=MC\cdot MD\)

giúp tôi ý d với bạn ơi

\(M=\dfrac{1}{1000}+\dfrac{1}{1002}+\dfrac{1}{1004}+...+\dfrac{1}{2000}\)

\(2M=\dfrac{1}{500}+\dfrac{1}{501}+\dfrac{1}{502}+...+\dfrac{1}{1000}\)

\(2M< \dfrac{1}{500}+\dfrac{1}{500}+\dfrac{1}{500}+...+\dfrac{1}{500}=\dfrac{500}{500}=1\)

\(M< \dfrac{1}{2}\)

\(\dfrac{3^{10}.15^5}{25^3.9^7}\)

\(=\dfrac{3^{10}.3^55^5}{\left(5^2\right)^3.\left(3^2\right)^7}\)

\(=\dfrac{3^{15}.5^5}{5^6.3^{14}}\)

\(=\dfrac{3.1}{5.1}\)

\(=\dfrac{3}{5}\)

Bài 4:

d: 

ĐKXĐ: \(x\notin\left\{1;-1;2;-2\right\}\)

\(\dfrac{x+4}{x-1}+\dfrac{x-4}{x+1}=\dfrac{x+8}{x-2}+\dfrac{x-8}{x+2}+6\)

=>\(\dfrac{\left(x+4\right)\left(x+1\right)+\left(x-4\right)\left(x-1\right)}{\left(x-1\right)\left(x+1\right)}=\dfrac{\left(x+8\right)\left(x+2\right)+\left(x-8\right)\left(x-2\right)+6\left(x-2\right)\left(x+2\right)}{\left(x-2\right)\left(x+2\right)}\)

=>\(\dfrac{2x^2+8}{\left(x-1\right)\left(x+1\right)}=\dfrac{2x^2+32+6x^2-24}{\left(x-2\right)\left(x+2\right)}\)

=>\(\dfrac{2x^2+8}{x^2-1}=\dfrac{8x^2+8}{x^2-4}\)

=>\(\left(2x^2+8\right)\left(x^2-4\right)=\left(8x^2+8\right)\left(x^2-1\right)\)

=>\(2x^4-32=8x^4-8\)

=>\(-6x^4=24\)

=>\(x^4=-4\left(loại\right)\)

Vậy: Phương trình vô nghiệm

c:

ĐKXĐ: \(x\notin\left\{-1;-3;-8;-10\right\}\)

 \(\dfrac{2}{x^2+4x+3}+\dfrac{5}{x^2+11x+24}+\dfrac{2}{x^2+18x+80}=\dfrac{9}{52}\)

=>\(\dfrac{2}{\left(x+1\right)\left(x+3\right)}+\dfrac{5}{\left(x+3\right)\left(x+8\right)}+\dfrac{2}{\left(x+8\right)\left(x+10\right)}=\dfrac{9}{52}\)

=>\(\dfrac{1}{x+1}-\dfrac{1}{x+3}+\dfrac{1}{x+3}-\dfrac{1}{x+8}+\dfrac{1}{x+8}-\dfrac{1}{x+10}=\dfrac{9}{52}\)

=>\(\dfrac{1}{x+1}-\dfrac{1}{x+10}=\dfrac{9}{52}\)

=>\(\dfrac{9}{\left(x+1\right)\left(x+10\right)}=\dfrac{9}{52}\)

=>(x+1)(x+10)=52

=>\(x^2+11x-42=0\)

=>(x+14)(x-3)=0

=>\(\left[{}\begin{matrix}x=-14\left(nhận\right)\\x=3\left(nhận\right)\end{matrix}\right.\)

b: 

ĐXKĐ: \(x\notin\left\{-2;-3;-4;-5;-6\right\}\)\(\dfrac{1}{x^2+5x+6}+\dfrac{1}{x^2+7x+12}+\dfrac{1}{x^2+9x+20}+\dfrac{1}{x^2+11x+30}=\dfrac{1}{8}\)

=>\(\dfrac{1}{\left(x+2\right)\left(x+3\right)}+\dfrac{1}{\left(x+3\right)\left(x+4\right)}+\dfrac{1}{\left(x+4\right)\left(x+5\right)}+\dfrac{1}{\left(x+5\right)\left(x+6\right)}=\dfrac{1}{8}\)

=>\(\dfrac{1}{x+2}-\dfrac{1}{x+3}+\dfrac{1}{x+3}-\dfrac{1}{x+4}+\dfrac{1}{x+4}-\dfrac{1}{x+5}+\dfrac{1}{x+5}-\dfrac{1}{x+6}=\dfrac{1}{8}\)

=>\(\dfrac{1}{x+2}-\dfrac{1}{x+6}=\dfrac{1}{8}\)

=>\(\dfrac{x+6-x-2}{\left(x+2\right)\left(x+6\right)}=\dfrac{1}{8}\)

=>(x+2)(x+6)=32

=>\(x^2+8x-20=0\)

=>(x+10)(x-2)=0

=>\(\left[{}\begin{matrix}x=-10\left(nhận\right)\\x=2\left(nhận\right)\end{matrix}\right.\)

a: \(\dfrac{x^2}{x^2+2x+2}+\dfrac{x^2}{x^2-2x+2}-\dfrac{4x^2-20}{x^4+4}=\dfrac{322}{65}\)

=>\(\dfrac{x^2\left(x^2-2x+2\right)+x^2\left(x^2+2x+2\right)-4x^2+20}{\left(x^2+2x+2\right)\left(x^2-2x+2\right)}=\dfrac{322}{65}\)

=>\(\dfrac{x^4-2x^3+2x^2+x^4+2x^3+2x^2-4x^2+20}{x^4+4}=\dfrac{322}{65}\)

=>\(\dfrac{2x^4+20}{x^4+4}=\dfrac{322}{65}\)

=>\(322\left(x^4+4\right)=65\left(2x^4+20\right)\)

=>\(322x^4+1288-130x^4-1300=0\)

=>\(192x^4=12\)

=>\(x^4=\dfrac{1}{16}\)

=>\(\left[{}\begin{matrix}x=\dfrac{1}{2}\left(nhận\right)\\x=-\dfrac{1}{2}\left(nhận\right)\end{matrix}\right.\)