Đáp án:

\(\frac{1}{4}+\frac{1}{12}+\frac{1}{24}+\frac{1}{40}+\frac{1}{60}+\frac{1}{84}=\frac{12}{7}\)

TK MK NHA CÁC BẠN MK ĐG BỊ ÂM ĐIỂM

đặt A =1/4+...+1/84

A=2/8 +2/24+...+2/168

A=2/2.4+2/4.6+...+2/12.14

A=1/2-1/4+1/4-1/6 +...+1/12-1/14

A=1/2-1/14

A=6/14=3/7

\(\frac{1}{4}+\frac{1}{12}+\frac{1}{24}+\frac{1}{40}+\frac{1}{60}+\frac{1}{84}\)

\(=\left(\frac{1}{4}+\frac{1}{12}\right)+\left(\frac{1}{24}+\frac{1}{40}\right)+\left(\frac{1}{60}+\frac{1}{84}\right)\)

\(=\frac{1}{3}+\frac{1}{15}+\frac{1}{35}=\frac{3}{7}\)

Áp dụng tc: \(\frac{1}{n}.\left(1+2+3+...+n\right)=\frac{1}{n}.\frac{n.\left(n+1\right)}{2}=\frac{n+1}{2}\)

=> H = \(\frac{1}{2}.2+\frac{1}{2}.3+\frac{1}{2}.4+...+\frac{1}{2}.85=\frac{1}{2}.\left(2+3+4+...+85\right)\)

        = \(\frac{1}{2}.\left(1+2+3+4+...+85-1\right)=\frac{1}{2}.\left(\frac{85.86}{2}-1\right)=\frac{1}{2}.3654=1827\)

Tính nhanh : 

\(A=\frac{1}{4}+\frac{1}{12}+\frac{1}{24}+\frac{1}{40}+\frac{1}{60}+\frac{1}{84}\)

\(A=2\left(\frac{1}{2\cdot4}+\frac{1}{4\cdot6}+\frac{1}{6\cdot8}+\frac{1}{8\cdot10}+\frac{1}{10\cdot12}+\frac{1}{12\cdot14}\right)\)

\(A=2\left(\frac{1}{2}-\frac{1}{4}+\frac{1}{4}-\frac{1}{6}+\frac{1}{6}-\frac{1}{8}+...+\frac{1}{10}-\frac{1}{12}+\frac{1}{12}-\frac{1}{14}\right)\)

\(A=2\left(\frac{1}{2}-\frac{1}{14}\right)\)

\(A=2\cdot\frac{3}{7}\)

\(A=\frac{6}{7}\)

\(A=\frac{1}{4}+\frac{1}{12}+\frac{1}{24}+\frac{1}{40}+\frac{1}{60}+\frac{1}{84}\)

\(A=\frac{2}{8}+\frac{2}{24}+\frac{2}{48}+\frac{2}{80}+\frac{2}{120}+\frac{2}{168}\)

\(A=\frac{2}{2.4}+\frac{2}{4.6}+\frac{2}{6.8}+\frac{2}{8.10}+\frac{2}{10.12}+\frac{2}{12.14}\)

\(A=\frac{1}{2}-\frac{1}{4}+\frac{1}{4}-\frac{1}{6}+\frac{1}{6}-\frac{1}{8}+\frac{1}{8}-\frac{1}{10}+\frac{1}{10}-\frac{1}{12}+\frac{1}{12}-\frac{1}{14}\)

\(A=\frac{1}{2}-\frac{1}{14}\)

\(A=\frac{3}{7}\)

_Chúc bạn học tốt_

A=1/4+1/12+1/24+1/40+1/60+1/84

A=2/8+2/24+2/48+2/80+2/120+2/168

A=1/2-1/4+1/4-1/6+1/6-1/8+1/8-1/10+1/10-1/12+1/12-1/14

A=1/2-1/14

A=7/14-1/14

A=6/14

A=3/7

84 ^2 + 22 ^2 -44* 84 -38 ^2

= 2400

\(84^2-44\cdot84+22^2-38^2\)

\(=\left(84-22\right)^2-38^2\)

\(=\left(62+38\right)\left(62-38\right)\)

\(=100\cdot24=2400\)

\(=log_35^2-log_350+log_36\)

\(=log_3\left(\dfrac{25}{50}\cdot6\right)=log_33=1\)

\(2\log_35-\log_350+\dfrac{1}{2}\log_336=\log_35^2-\log_350+\log_336^{\dfrac{1}{2}}=\log_325-\log_350+\log_36=\log_3\left(\dfrac{25}{50}.6\right)=\log_33=1\)

a)

- Oxit bazo:

+ CuO: đồng oxit

+ CaO: canxi oxit

- Oxit axit: + SO3 : lưu huỳnh trioxit

- Bazo:

+ Ba(OH)2: bari hydroxit

- Axit:

+ HNO3: axit nitric

+ HCl: axit clohiric

- Muối:

+ CaCO3: Canxi cacbonat

+ AgNO3: Bạc nitrat

+ Na2CO3: natri cacbonat

b)

- Chất td với nước: CaO, Ba(OH)2, SO3, Na2CO3, CaCO3, HCl, HNO3.

- H2SO4: CuO, CaO, Ba(OH)2, Na2CO3, AgNO3, CaCO3.

- dd KOH: SO3, HNO3, HCl, Na2CO3, AgNO3, CaCO3