Em xem lại ý nhận biết 3 chất cuối sao đầu CH₂ lại là liên kết 3 được nhỉ

B

B

Bài 10:

$-A=\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+\frac{1}{90}$

$=\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+...+\frac{1}{9.10}$

$=\frac{5-4}{4.5}+\frac{6-5}{5.6}+\frac{7-6}{6.7}+...+\frac{10-9}{9.10}$

$=\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+....+\frac{1}{9}-\frac{1}{10}$

$=\frac{1}{4}-\frac{1}{10}=\frac{3}{20}$

$\Rightarrow A=\frac{-3}{20}$

Bài 11:

$A=\frac{2n}{n+3}=\frac{2(n+3)-6}{n+3}=2-\frac{6}{n+3}$
Để $A$ nguyên thì $\frac{6}{n+3}$ nguyên.

Với $n$ nguyên thì điều trên xảy ra khi $6\vdots n+3$

$\Rightarrow n+3\in\left\{\pm 1; \pm 2; \pm 3; \pm 6\right\}$

$\Rightarrow n\in\left\{-4; -2; -1; -5; -6; 0; -9; 3\right\}$

e: \(E=\frac{8}{77}+\frac{12}{187}+\frac{4}{323}+\frac{8}{21}+\frac{10}{456}\)

\(=\frac{8}{7\cdot11}+\frac{12}{11\cdot17}+\frac{4}{17\cdot19}+\frac{10}{19\cdot24}+\frac{8}{21}\)

\(=2\left(\frac{4}{7\cdot11}+\frac{6}{11\cdot17}+\frac{2}{17\cdot19}+\frac{5}{19\cdot24}\right)+\frac{8}{21}\)

\(=2\left(\frac17-\frac{1}{11}+\frac{1}{11}-\frac{1}{17}+\frac{1}{17}-\frac{1}{19}+\frac{1}{19}-\frac{1}{24}\right)+\frac{8}{21}\)

\(=2\left(\frac17-\frac{1}{24}\right)+\frac{8}{21}=2\cdot\frac{24-7}{7\cdot24}+\frac{8}{21}=2\cdot\frac{17}{7\cdot24}+\frac{8}{21}\)

\(=\frac{17}{7\cdot12}+\frac{8}{21}=\frac{17}{84}+\frac{32}{84}=\frac{49}{84}=\frac{7}{12}\)

f: \(G=\frac{2}{35}-\frac{3}{28}+\frac{3}{10}+\frac{7}{78}-\frac{9}{52}\)

\(=\frac{2}{5\cdot7}-\frac{3}{4\cdot7}+\frac{3}{2\cdot5}+\frac{7}{78}-\frac{9}{4\cdot13}\)

\(=\frac15-\frac17-\frac14+\frac17+\frac12-\frac15+\frac{7}{78}-\frac14-\frac{1}{13}\)

\(=\left(\frac15-\frac15\right)+\left(-\frac17+\frac17\right)+\left(-\frac14+\frac12-\frac14\right)+\frac{7}{78}-\frac{1}{13}\)

\(=\frac{7}{78}-\frac{1}{13}=\frac{7}{78}-\frac{6}{78}=\frac{1}{78}\)

a: Thay x=0 và y=5 vào (d), ta được:

m*0+5=5

=>5=5(đúng)

=>ĐPCM

b: x1<x2; |x1|>|x2|

=>x1*x2<0

PTHĐGĐ là:

x^2-mx-5=0

Vì a*c<0

nên x1,x2 luôn trái dấu

=>Với mọi m

Ta thay x bằng số -4. Khi đó -4+4=0, mà 0 mũ 2020 thì vẫn bằng 0. 0+17=17. Đáp án: 17

`-3/7xx15/13-3/7xx11/13-3/7`

`=-3/7xx(15/13+11/13+1)`

`=-3/7xx3`

`=-9/7`