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CHƯA HỌC ĐẾN VẬY BN HỎI LÀM J

xl nha. mk chua hk  toi. ra tet mk moi hk den bai nay

m ở đâu vậy bạn

\(a.\left[x\cdot\left(x+1\right)\right]:2=136\\ x\cdot\left(x+1\right)=136\cdot2\\ x\cdot\left(x+1\right)=2\cdot2\cdot2\cdot17\cdot2\\ x\cdot\left(x+1\right)=\left(2\cdot2\cdot2\cdot2\right)\cdot17\\ x\cdot\left(x+1\right)=16\cdot17\\ =>x=16\\ b.\left[x\cdot\left(x+1\right)\right]:2=300\\ x\cdot\left(x+1\right)=3\cdot2\cdot2\cdot5\cdot5\cdot2\\ x\cdot\left(x+1\right)=\left(3\cdot2\cdot2\cdot2\right)\cdot\left(5\cdot5\right)\\ x\cdot\left(x+1\right)=24\cdot25\\ =>x=24\\ c.\left[x\cdot\left(x+1\right)\right]:2=561\\ x\cdot\left(x+1\right)=2\cdot3\cdot11\cdot17\\ x\cdot\left(x+1\right)=\left(17\cdot2\right)\cdot\left(3\cdot11\right)\\ x\cdot\left(x+1\right)=34\cdot33\\ =>x=33\)

A= 1/3+1/6+1/10+...+1/561

  = 2. (1/6+1/12+1/20+...+1/1122)

  = 2. [1/(2.3) + 1/(3.4) + 1/(4.5) +...+1/(33.34)]

  = 2. ( 1/2 - 1/3 +1/3 - 1/4 + 1/4 - 1/5 +...+ 1/33 - 1/34 )

  =2. (1/2 - 1/34)

  =2. 8/17

  =16/17

Vì 16/17 > 16/18 = 8/9 -> A > 8/9

\(A=\frac{1}{3}+\frac{1}{6}+\frac{1}{10}+...+\frac{1}{561}\)

\(A=\frac{2}{6}+\frac{2}{12}+\frac{2}{20}+...+\frac{2}{1122}\)

\(A=\frac{2}{2.3}+\frac{2}{3.4}+\frac{2}{4.5}+...+\frac{2}{33.34}\)

\(A=2.\left(\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{33.34}\right)\)

\(A=2.\left(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{33}-\frac{1}{34}\right)\)

\(A=2.\left(\frac{1}{2}-\frac{1}{34}\right)\)

\(A=2.\left(\frac{17-1}{34}\right)\)

\(A=2.\frac{8}{17}\)

\(A=\frac{16}{17}>\frac{16}{18}=\frac{8}{9}\)

\(\Rightarrow A>\frac{8}{9}\)

a) (Bạn tự vẽ hình ạ)

Ta có AD.AB = AE.AC

⇒ \(\dfrac{AD}{AC}=\dfrac{AE}{AB}\)

Xét \(\Delta ABC\) và \(\Delta AED\) có:

\(\dfrac{AD}{AC}=\dfrac{AE}{AB}\)

\(\widehat{A}:chung\)

⇒ \(\Delta ABC\sim\Delta AED\)   \(\left(c.g.c\right)\)

⇒ DE // BC

b)