Có \(\left(a-b\right)^2+\left(b-c\right)^2+\left(c-a\right)^2=4\left(a^2+b^2+c^2-ab-ac-bc\right)\)

\(\Rightarrow a^2-2ab+b^2+b^2-2bc+c^2+c^2-2ac+a^2-4a^2-4b^2-4c^2+4ab+4ac+4bc=0\)

\(\Rightarrow-2a^2-2b^2-2c^2+2ab+2ac+2bc=0\)

\(\Rightarrow-\left(a^2-2ab+b^2\right)-\left(b^2-2bc+c^2\right)-\left(a^2-2ac+c^2\right)=0\)

\(\Rightarrow-\left(a-b\right)^2-\left(b-c\right)^2-\left(a-c\right)^2=0\)

\(\Rightarrow\left[{}\begin{matrix}\left(a-b\right)^2=0\\\left(b-c\right)^2=0\\\left(a-c\right)^2=0\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}a-b=0\\b-c=0\\a-c=0\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}a=b\\b=c\\a=c\end{matrix}\right.\Rightarrow a=b=c\left(đpcm\right)\)

\(a^2+b^2+1\ge ab+a+b\)

\(<=>2a^2+2b^2+2\geq 2ab+2a+2b\\<=>(a^2-2ab+b^2)+(a^2-2a+1)+(b^2-2b+1)\geq 0\\<=>(a-b)^2+(a-1)^2+(b-1)^2\geq 0\)

$\Rightarrow $ \(a^{2}+b^{2}\geq 2ab\) (1)

$\Rightarrow $ \(a^{2}+1\geq 2a\) (2)

$\Rightarrow $ \(b^{2}+1\geq 2b\) (3)

(1), (2) và (3)\(\Rightarrow a^{2}+b^{2}+1\geq ab+a+b\)

a: a^3+b^3+c^3-3abc

=(a+b)^3+c^3-3ab(a+b)-3bac

=(a+b+c)(a^2+2ab+b^2-ac-bc+c^2)-3ab(a+b+c)

=(a+b+c)(a^2+b^2+c^2-ab-ac-bc)

b: Đề sai rồi bạn

c: 2(a+b+c)*(b/2+c/2-a/2)

=(a+b+c)(b+c-a)

=(b+c)^2-a^2

=c^2+2bc+c^2-a^2

Ta có : a>b

Cộng 2 vế : a+2>b+2 =>đpcm

a>b

nên a+2>b+2

Đặt \(\dfrac{a}{b}=\dfrac{c}{d}=k\Rightarrow a=bk,c=dk\)

a) \(\dfrac{a^2-b^2}{c^2-d^2}=\dfrac{b^2k^2-b^2}{d^2k^2-d^2}=\dfrac{b^2}{d^2}\)\(=\dfrac{\dfrac{a}{k}.b}{\dfrac{c}{k}.d}=\dfrac{ab}{cd}=VT\)

Vậy...

b) \(\dfrac{5a+3b}{5a-3b}=\dfrac{5bk+3b}{5bk-3b}=\dfrac{5k+3}{5k-3}\)

\(\dfrac{5c+3d}{5c-3d}=\dfrac{5dk+3d}{5dk-3d}=\dfrac{5k+3}{5k-3}\)

Suy ra \(\dfrac{5a+3b}{5a-3b}=\dfrac{5c+3d}{5c-3d}\)

c) \(\dfrac{7a^2+3ab}{11a^2-8b^2}=\dfrac{7\left(bk\right)^2+3\left(bk\right).b}{11\left(bk\right)^2-8b^2}\)\(=\dfrac{7k^2+3k}{11k^2-8}\)

\(\dfrac{7c^2+3cd}{11c^2-8d^2}=\dfrac{7\left(dk\right)^2+3\left(dk\right).d}{11\left(dk\right)^2-8d^2}=\dfrac{7k^2+3k}{11k^2-8}\)

Suy ra \(\dfrac{7a^2+3ab}{11a^2-8b^2}=\dfrac{7c^2+3cd}{11c^2-8d^2}\)

a) Có: \(\dfrac{a}{b}=\dfrac{c}{d}\)

=> \(ad=bc\)

=> \(\dfrac{a}{c}=\dfrac{b}{d}\) => \(\left(\dfrac{a}{c}\right)^2=\left(\dfrac{b}{d}\right)^2=\dfrac{ab}{cd}=\dfrac{a^2}{c^2}=\dfrac{b^2}{d^2}=\dfrac{a^2-b^2}{c^2-d^2}\)

(theo tính chất dãy tỉ số bằng nhau)

=> (đpcm)

b) Có: \(\dfrac{a}{b}=\dfrac{c}{d}\) => \(\dfrac{a}{c}=\dfrac{b}{d}\)

=> \(\dfrac{5a}{5c}=\dfrac{3b}{3d}=\dfrac{5a+3b}{5c+3d}=\dfrac{5a-3b}{5c-3d}\)(theo tính chất dãy tỉ số bằng nhau)

=> \(\dfrac{5a+3b}{5a-3b}=\dfrac{5c+3d}{5c-3d}\) (đpcm)

c) Có: \(\dfrac{a}{b}=\dfrac{c}{d}\Leftrightarrow\dfrac{a}{c}=\dfrac{b}{d}\)

=> \(\dfrac{a^2}{c^2}=\dfrac{ab}{cd}=\dfrac{b^2}{d^2}\)          => \(\dfrac{7a^2}{7c^2}=\dfrac{3ab}{3cd}=\dfrac{11a^2}{11c^2}=\dfrac{8b^2}{8d^2}\)

=> \(\dfrac{7a^2+3ab}{7c^2+3cd}=\dfrac{11a^2-8b^2}{11c^2-8d^2}\) (theo tính chất dãy tỉ số bằng nhau)

=> \(\dfrac{7a^2+3ab}{11a^2-8b^2}=\dfrac{7c^2+3cd}{11c^2-8d^2}\)(đpcm)

#Ayumu

\(\Rightarrow\left\{{}\begin{matrix}\left(a-b\right)^2=0\\\left(b-c\right)^2=0\\\left(c-a\right)^2=0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}a=b\\b=c\\c=a\end{matrix}\right.\)

\(\Rightarrow a=b=c\left(đpcm\right)\)

Ta có: \(\left(a+b+c\right)^2=a^2+b^2+c^2+2ab+2bc+2ac\)

\(3\left(a^2+b^2+c^2\right)=3a^2+3b^2+3c^2\)
mà \(\left(a+b+c\right)^2=3\left(a^2+b^2+c^2\right)\Rightarrow a^2+b^2+c^2+2ab+2bc+2ca=3a^2+3b^2+3c^2\)

\(\Leftrightarrow2a^2+2b^2+2c^2-2ab-2bc-2ca=0\)
\(\Leftrightarrow a^2-2ab+b^2+b^2-2bc+c^2+c^2-2ac+a^2=0\)
\(\Leftrightarrow\left(a-b\right)^2+\left(b-c\right)^2+\left(c-a\right)^2=0\)

Vì \(\left\{{}\begin{matrix}\left(a-b\right)^2\ge0\forall a,b\\\left(b-c\right)^2\ge0\forall b,c\\\left(c-a\right)^2\ge0\forall a,c\end{matrix}\right.\)

Mà \(\left(a-b\right)^2+\left(b-c\right)^2+\left(c-a\right)^2=0\)
\(\Rightarrow\left\{{}\begin{matrix}\left(a-b\right)^2=0\\\left(b-c\right)^2=0\\\left(c-a\right)^2=0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}a=b\\b=c\\c=a\end{matrix}\right.\Leftrightarrow}a=b=c\Rightarrowđpcm}\)

b: Áp dụng hệ thức lượng trong tam giác vuông vào ΔABH vuông tại H có HE là đường cao ứng với cạnh huyền AB, ta được:

\(BE\cdot BA=BH^2\)

hay \(BE=\dfrac{BH^2}{BA}\)

Áp dụng hệ thức lượng trong tam giác vuông vào ΔACH vuông tại H có HF là đường cao ứng với cạnh huyền CA, ta được:

\(CF\cdot CA=CH^2\)

hay \(CF=\dfrac{CH^2}{CA}\)

Ta có: \(\dfrac{BE}{CF}=\dfrac{BH^2}{AB}:\dfrac{CH^2}{CA}\)

\(=\dfrac{BH^2}{CH^2}\cdot\dfrac{AC}{AB}\)

\(=\dfrac{AB^4\cdot AC}{AC^4\cdot AC}=\dfrac{AB^3}{AC^3}\)

3: Ta có \(\dfrac{1}{u_{n+1}}=\dfrac{1}{u_n}-1\).

Do đó \(\dfrac{1}{u_{100}}=\dfrac{1}{u_{99}}-1=\dfrac{1}{u_{98}}-2=...=\dfrac{1}{u_1}-99=\dfrac{1}{-2}-99=\dfrac{-199}{2}\Rightarrow u_{100}=\dfrac{-2}{199}\).