Bài 1:

\(\dfrac{1}{n}-\dfrac{1}{n+1}=\dfrac{n+1}{n\left(n+1\right)}-\dfrac{n}{\left(n+1\right)n}\)

\(=\dfrac{n+1-n}{n\left(n+1\right)}=\dfrac{1}{n\left(n+1\right)}\)

b)A=\(\dfrac{1}{1.2}+\dfrac{1}{2.3}+\dfrac{1}{3.4}+...+\dfrac{1}{9.10}\)

\(A=1\left(1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{9}-\dfrac{1}{10}\right)\)

\(\Rightarrow A=1\left(1-\dfrac{1}{10}\right)\)

A=\(1\left(\dfrac{10}{10}-\dfrac{1}{10}\right)\)

\(\Rightarrow A=\dfrac{9}{10}\)

Vậy A=\(\dfrac{9}{10}\)

~ chúc bạn học tốt~

muốn bài nào mk giúp!!!

câu trả lời của mk là:

câu hỏi gì mà kì cục quá!!

CHỊU

\(\frac{x}{5}=\frac{y}{3}=>\frac{x}{5}^2=\frac{y}{3}^2=>\frac{x^2}{25}=\frac{y^2}{9}\)

Áp dụng tính chất của dãy tỉ số bằng nhau ta có:

\(\frac{x^2}{25}=\frac{y^2}{9}=\frac{x^2-y^2}{25-9}=\frac{4}{16}=\frac{1}{4}\)

=>\(x^2=25.\frac{1}{4}=\frac{25}{4}=>x=\frac{5}{2},-\frac{5}{2}\)

Xét \(x=\frac{5}{2}\)=>\(\frac{x}{5}=\frac{y}{3}=\frac{\frac{5}{2}}{5}=\frac{1}{2}=>y=\frac{1}{2}.3=\frac{3}{2}\)

Xét \(x=-\frac{5}{2}\)=>\(\frac{x}{5}=\frac{y}{3}=\frac{-\frac{5}{2}}{5}=-\frac{1}{2}=>y=-\frac{1}{2}.3=-\frac{3}{2}\)

\(\left(-25\%+0,75+\dfrac{7}{12}\right)\div\left(-2\dfrac{1}{8}\right)\)

=\(\left(\dfrac{-1}{4}+\dfrac{3}{4}+\dfrac{7}{12}\right)\div\left(\dfrac{-17}{8}\right)\)

=\(\left(\dfrac{-3}{12}+\dfrac{9}{12}+\dfrac{7}{12}\right).\dfrac{-8}{17}\)

=\(\dfrac{13}{12}.\dfrac{-8}{17}=\dfrac{-26}{51}\)

\(-2\dfrac{1}{4}.\)\(\left(3\dfrac{5}{12}-1\dfrac{2}{9}\right)\)

=\(\dfrac{-9}{4}\).\(\left(\dfrac{41}{12}-\dfrac{11}{9}\right)\)

=\(\dfrac{-9}{4}.\dfrac{41}{12}-\dfrac{-9}{4}.\dfrac{11}{9}\)

=\(\dfrac{-123}{16}-\dfrac{-11}{4}\)

=\(\dfrac{-123}{16}-\dfrac{-44}{16}\)

=\(\dfrac{-79}{16}\)

\(\left|3x-\frac{7}{6}\right|\ge0\Rightarrow5,2-\left|3x-\frac{7}{6}\right|\le5,2\)

Dấu "=" xảy ra khi và chỉ khi:

\(3x-\frac{7}{6}=0\)

\(\Rightarrow3x=\frac{7}{6}\)

\(\Rightarrow x=\frac{7}{6}:3=\frac{7}{18}\)

Vậy GTLN của biểu thức là 5,2 <=> x=7/18

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

=a+b-c+a-b+c-b-c+a-a+b+c

=a.3-a.1+b.2-b.2-c.2+c.2

5963 = 5.1000 + 9.100 + 6.10 + 3.1 = 5.10³ + 9.10² + 6.10¹ + 3.10⁰

abcde = a.10000 + b.1000 + c.100 + d.10 + e.1 = a.10⁴ + b.10³ + c.10² + d.10¹ + e.10⁰

nè bn!!!!

máy hết chỗ đánh làm ơn chuyển 3 dòng đầu này xuống cuối

\(\left(\dfrac{20}{9}-x\right)=\dfrac{1}{3}-\dfrac{1}{9}\)

\(\left(\dfrac{20}{9}-x\right)=\dfrac{2}{9}\)

\(x=\dfrac{20}{9}-\dfrac{2}{9}\)

\(x=2\)

vậy x=2

giúp mk nha !!!THANKS!!!