A) X₍₂₎ =1+1=2=1₍₂₎

_{=> X = 1}

_B) X₍₈₎ = 4+4=8 =1₍₈₎

 => X =1

AB+BC+CD+AD=66 (1)

AB+BC+AC=56 (2)   =>(AB+BC)=56-AC (*)       

AC+CD+AD=60 (3)     => CD+AD=60-AC (**)    

(1)(2)(3)

2(AB+BC+CD+AD)=66+56+60=182

AB+BC+CD+AD=91 (4)

(4);(*)(**) => 60-AC+56-AC=91 => AC= 25/2= 12,5

30 dm = 300 cm                             100 dm = 10 m

30 m = 3000 cm                             300 cm = 3 m

40 cm = 4 dm                                20 m = 200 dm

tự ra đề thì phải tự làm. 

chưa ai làm bt  ntn  cả.

tự làm  đê.

Q=\(\left\{\frac{a}{b};b\ne0\right\}\)

\(x-y=\left(5+\frac{3}{13}+\frac{7}{26}+\frac{1}{2}\right)-\left(5+\frac{2}{3}+\frac{4}{37}+\frac{5}{111}\right)\)

\(=\left(\frac{1}{2}-\frac{2}{3}\right)+\left(\frac{3}{13}-\frac{4}{37}\right)+\left(\frac{7}{26}-\frac{5}{111}\right)>0\)

=>  x> y

\(M=\frac{x+y}{xy}.\frac{1}{z}\ge\frac{2\sqrt{xy}}{xy}.\frac{1}{z}=\frac{2}{z\sqrt{xy}}\ge\frac{2}{z\left(\frac{x+y}{2}\right)}=\frac{4}{z\left(x+y\right)}\)

\(=\frac{4}{z\left(1-z\right)}=\frac{4}{\frac{1}{4}-\left(z-\frac{1}{2}\right)^2}\ge16\)

Min M= 16 khi  z=1/2 và  x=y =1/4.

a) \(\sqrt{9-4\sqrt{5}}-\sqrt{5}=\sqrt{\left(\sqrt{5}-2\right)^2}-\sqrt{5}=\left|\sqrt{5}-2\right|-\sqrt{5}=\sqrt{5}-2-\sqrt{5}=-2\)

b) \(\left(4-\sqrt{7}\right)^2=4^2-2.4.\sqrt{7}+\sqrt{7}^2=16-8\sqrt{7}+7=23-8\sqrt{7}\)

c)  \(\sqrt{23+8\sqrt{7}}=\sqrt{\left(4+\sqrt{7}\right)^2}=\left|4+\sqrt{7}\right|=\sqrt{7}+4\)