Bài 10: Phép nhân phân số

ngày thứ 1 bạn đọc số trang là

80 x 2/5= 32 (trang)

ngày thứ 2 bạn đọc số trang là

80-32= 48 (trang)

48

Ngày thứ 1 bạn đọc số trang là

80 x 2/5= 32 (trang)

Ngày thứ 2 bạn đọc số trang là

80-32= 48 (trang)

\(S=\dfrac{1}{1.2}+\dfrac{1}{2.3}+...+\dfrac{1}{2004.2005}\\ =1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+...+\dfrac{1}{2004}-\dfrac{1}{2005}\\ =1-\dfrac{1}{2005}\\ =\dfrac{2004}{2005}\)

a: \(\dfrac{-2}{3}\cdot\dfrac{?}{4}=\dfrac{1}{2}\)

=>\(\dfrac{?}{4}=\dfrac{1}{2}:\dfrac{-2}{3}=\dfrac{1}{2}\cdot\dfrac{-3}{2}=\dfrac{-3}{4}\)

=>?=-3

b: \(\dfrac{?}{3}\cdot\dfrac{5}{8}=-\dfrac{5}{12}\)

=>\(\dfrac{?}{3}=-\dfrac{5}{12}:\dfrac{5}{8}=\dfrac{-8}{12}=-\dfrac{2}{3}\)

=>?=-2

c: \(\dfrac{5}{6}\cdot\dfrac{3}{?}=\dfrac{1}{4}\)

=>\(\dfrac{3}{?}=\dfrac{1}{4}:\dfrac{5}{6}=\dfrac{1}{4}\cdot\dfrac{6}{5}=\dfrac{6}{20}=\dfrac{3}{10}\)

=>?=10

\(a,12.\dfrac{-7}{11}.\dfrac{5}{6}.\dfrac{22}{7}=\left(12.\dfrac{5}{6}\right)\left(\dfrac{-7}{11}.\dfrac{22}{7}\right)=10.\left(-2\right)=-20\\ b,\dfrac{-8}{15}.\dfrac{7}{9}.\dfrac{5}{8}.\left(-18\right)=\left(\dfrac{-8}{15}.\dfrac{5}{8}\right)\left[\dfrac{7}{9}.\left(-18\right)\right]=\dfrac{-1}{3}.\left(-14\right)=\dfrac{14}{3}\)

a) 12.−711.56.227= (12.56)(−711.227)= 10.(−2)= −20.

b) −815.79.58.(−18)= (−815.58)[79.(−18)]= −13.(−14)= 143.

\(=\dfrac{3.4.5...101}{2.3.4...100}=\dfrac{101}{2}\)

\(\left(1+\dfrac{1}{2}\right)\times\left(1+\dfrac{1}{3}\right)\times...\times\left(1+\dfrac{1}{100}\right)\\ =\dfrac{3}{2}\times\dfrac{4}{3}\times...\times\dfrac{101}{100}\)

\(=\dfrac{101}{2}\)

\(A=\dfrac{1}{3.4}+\dfrac{1}{4.5}+...+\dfrac{1}{19.20}=\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+...+\dfrac{1}{19}-\dfrac{1}{20}=\dfrac{1}{3}-\dfrac{1}{20}< \dfrac{1}{3}\)

\(B=\dfrac{2}{3.4}+\dfrac{2}{4.5}+...+\dfrac{2}{19.20}=2\left(\dfrac{1}{3.4}+\dfrac{1}{4.5}+...+\dfrac{1}{19.20}\right)=2\left(\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+...+\dfrac{1}{19}-\dfrac{1}{20}\right)=2\left(\dfrac{1}{3}-\dfrac{1}{20}\right)=\dfrac{2}{3}-\dfrac{2}{20}< \dfrac{2}{3}\)

\(C=\dfrac{1}{2.4}+\dfrac{1}{4.6}+...+\dfrac{1}{18.20}=\dfrac{1}{2}\left(\dfrac{2}{2.4}+\dfrac{2}{4.6}+...+\dfrac{2}{18.20}\right)=\dfrac{1}{2}\left(\dfrac{1}{2}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{6}+...+\dfrac{1}{18}-\dfrac{1}{20}\right)=\dfrac{1}{2}\left(\dfrac{1}{2}-\dfrac{1}{20}\right)=\dfrac{1}{4}-\dfrac{1}{40}< \dfrac{1}{4}\)

\(D=\dfrac{1}{4.6}+\dfrac{1}{6.8}+...+\dfrac{1}{18.20}=\dfrac{1}{2}\left(\dfrac{2}{4.6}+\dfrac{2}{6.8}+...+\dfrac{2}{18.20}\right)=\dfrac{1}{2}\left(\dfrac{1}{4}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{8}+...+\dfrac{1}{18}-\dfrac{1}{20}\right)=\dfrac{1}{2}\left(\dfrac{1}{4}-\dfrac{1}{20}\right)=\dfrac{1}{8}-\dfrac{1}{40}< \dfrac{1}{8}\)

d: \(=\dfrac{-5}{9}+\dfrac{5}{9}:\dfrac{20-25}{12}\)

\(=\dfrac{-5}{9}+\dfrac{5}{9}\cdot\dfrac{12}{-5}=\dfrac{-5}{9}-\dfrac{5}{9}\cdot\dfrac{12}{5}=\dfrac{-5}{9}\cdot\dfrac{17}{5}=-\dfrac{17}{9}\)

e: \(=\dfrac{1}{7}-\dfrac{1}{7}-3\cdot\dfrac{4}{3}\cdot4=-16\)

f: \(=\dfrac{3}{2}\cdot\dfrac{4}{3}\cdot...\cdot\dfrac{100}{99}=\dfrac{100}{2}=50\)

\(\dfrac{-8}{15}.\dfrac{3}{7}.\dfrac{15}{-8}.14=\left(\dfrac{-8}{15}.\dfrac{15}{-8}\right).\left(\dfrac{3}{7}.14\right)=1.6=6\)

\(\dfrac{17}{27}.\dfrac{13}{9}-\dfrac{4}{9}.\dfrac{17}{27}=\dfrac{17}{27}.\left(\dfrac{13}{9}-\dfrac{4}{9}\right)=\dfrac{17}{27}.1=\dfrac{17}{27}\)

1727.139−49.1727=1727.(139−49)=1727.1=1727

a: Tia OP và OQ

b: Tia PQ và tia Px

c: Điểm P và Q

d: Điểm O và Q

a: =>4x-2=3x+9

hay x=11

b: =>-15+3x=x-3

=>2x=12

hay x=6

c: =>-3x+3=4x+10

=>-7x=7

hay x=-1