Ta có: 

Đa thức: \(f\left(x\right)=ax^2+bx+c\) ⋮ 5

\(\Rightarrow f\left(x\right)=5\cdot\left(\dfrac{a}{5}x^2+\dfrac{b}{5}x+\dfrac{c}{5}\right)\) ⋮ 5

\(\Rightarrow a,b,c\in B\left(5\right)\) 

Vậy khi f(x) chia hết cho 5 thì a,b,c chia hết cho 5

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\(\left(-\dfrac{3}{4}+\dfrac{2}{5}\right):\dfrac{2}{7}+\left(\dfrac{3}{5}+\dfrac{1}{4}\right):\dfrac{3}{7}\)

\(=\left(-\dfrac{15}{20}+\dfrac{8}{20}\right):\dfrac{2}{7}+\left(\dfrac{12}{20}+\dfrac{5}{20}\right):\dfrac{3}{7}\)

\(=\left(-\dfrac{7}{20}\right):\dfrac{2}{7}+\dfrac{17}{20}:\dfrac{3}{7}\)

\(=-\dfrac{7}{20}.\dfrac{7}{2}+\dfrac{17}{20}.\dfrac{7}{3}\)

\(=\dfrac{7}{20}\left(-\dfrac{7}{2}+\dfrac{17}{3}\right)\)

\(=\dfrac{7}{20}\left(-\dfrac{21}{6}+\dfrac{34}{6}\right)\)

\(=\dfrac{7}{20}.\dfrac{13}{6}\)

\(=\dfrac{91}{120}\)

( -3/4 + 2/5 ) : 2/7 + ( 3/5 + 1/4 ) : 3/7 = 91/120

\(\dfrac{3}{7}.\dfrac{9}{36}-\dfrac{1}{14}.\dfrac{1}{13}\)

\(=\dfrac{3.9}{7.36}-\dfrac{1.1}{14.13}\)

\(=\dfrac{3}{28}-\dfrac{1}{182}=\dfrac{37}{364}\)

3/7 . 9/36 - 1/14 . 1/13

= 3/28 - 1/182 

= 37/364

\(A=\left(7-\dfrac{3}{4}+\dfrac{1}{3}\right)-\left(6+\dfrac{5}{4}-\dfrac{4}{3}\right)-\left(5-\dfrac{7}{4}+\dfrac{5}{3}\right)\)

\(\Rightarrow A=7-\dfrac{3}{4}+\dfrac{1}{3}-6-\dfrac{5}{4}+\dfrac{4}{3}-5+\dfrac{7}{4}-\dfrac{5}{3}\)

\(\Rightarrow A=\left(7-6-5\right)-\dfrac{3}{4}-\dfrac{5}{4}+\dfrac{7}{4}+\dfrac{1}{3}+\dfrac{4}{3}-\dfrac{5}{3}\)

\(\Rightarrow A=-4-\dfrac{1}{4}+0\)

\(\Rightarrow A=-\dfrac{17}{4}\)

Cách 2 tính trong dấu ngoặc đơn của A, bạn tự tính nhé.

A = -17/4

\(Ư\left(48\right)=\left\{1;-1;-2;2;3;-3;6;-6;8;-8;12;-12;-24;24;16;-16;48;-48\right\}\\ Ư\left(8\right)=\left\{1;-1;2;-2;-4;4;8;8\right\}\\ Ư\left(14\right)=\left\{-1;1;2;-2;7;-7;14;-14\right\}\\ Ư\left(20\right)=\left\{-1;1;2;-2;5;-5;4;-4;-10;10;-20;20\right\}\)

\(\dfrac{3}{5}+\dfrac{-1}{25}-\dfrac{8}{20}\)

\(=\dfrac{15}{25}+\dfrac{-1}{25}-\dfrac{8}{20}\)

\(=\dfrac{14}{25}-\dfrac{2}{5}\)

\(=\dfrac{14}{25}-\dfrac{10}{25}\)

\(=\dfrac{4}{25}\)

\(=\dfrac{15}{100}+\dfrac{-4}{100}-\dfrac{40}{100}\)

\(=\dfrac{-29}{100}\)

\(A=\dfrac{1}{2\cdot5}+\dfrac{1}{5\cdot8}+...+\dfrac{1}{95\cdot98}\)

\(3A=\dfrac{3}{2\cdot5}+\dfrac{3}{5\cdot8}+...+\dfrac{3}{95\cdot98}\)

\(3A=\dfrac{1}{2}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{8}+...+\dfrac{1}{95}-\dfrac{1}{98}\)

\(3A=\dfrac{1}{2}-\dfrac{1}{98}\)

\(A=\dfrac{\dfrac{1}{2}-\dfrac{1}{98}}{3}=\dfrac{8}{49}\)

a -35/50 = -7/10

b  510/2805 = 2/11

c  119/126

B2

-2/3= -8/12 , -1/4= -3/12

-8/12<-3/12 nên -2/3<-1/4

b 2/3  5/6

12/18 và 15/18

12/18<15/18

nên 14/21<60/72

bài 1 :

a) = -7/10

b) = 510/2805 = 2/11

c) = 17/18

= ( 7²⁰²⁴ + 32 ). ( 7¹⁰¹² . 7¹⁰¹² + 34 )

= ( 7²⁰²⁴ + 32 ) . ( 7²⁰²⁴ + 34 )

= 7²⁰²⁴ ( 32 + 34 )

= 7²⁰²⁴ . 66  ⋮ 6