Bài 7: Phép cộng phân số

1) \(x:3\dfrac{1}{15}=1\dfrac{1}{2}\)

    \(x:\dfrac{46}{15}=\dfrac{3}{2}\)

    \(x=\dfrac{3}{2}\cdot\dfrac{46}{15}\)

    \(x=\dfrac{23}{5}\)

2) \(5\dfrac{4}{7}:x=13\)

    \(\dfrac{39}{7}:x=13\) 

            \(x=\dfrac{39}{7}:13\)

            \(x=\dfrac{39}{7}:\dfrac{13}{1}\)

            \(x=\dfrac{39}{7}\cdot\dfrac{1}{13}\)

            \(x=\dfrac{3}{7}\)

3) \(7x-3x=3,2\)

      \(\left(7-3\right)x=\dfrac{16}{5}\)

          \(4x=\dfrac{16}{5}\)

            \(x=\dfrac{16}{5}:4\)

            \(x=\dfrac{4}{5}\)

1) \(\dfrac{2}{5}-\dfrac{1}{4}+\dfrac{3}{10}=\dfrac{8}{20}-\dfrac{5}{20}+\dfrac{6}{20}=\dfrac{9}{20}\)

2) \(-\dfrac{2}{5}:\left(1-\dfrac{1}{10}\right)=-\dfrac{2}{5}:1-\dfrac{1}{10}=-\dfrac{2}{5}-\dfrac{1}{10}=-\dfrac{1}{2}\)

3) \(\dfrac{7}{8}\cdot\dfrac{4}{9}+\dfrac{1}{14}:\dfrac{5}{14}=\dfrac{7}{18}+\dfrac{1}{5}=\dfrac{35}{90}+\dfrac{18}{90}=\dfrac{53}{90}\)

8)3

9)6

10)23/7

\(8)\) \(( -6,2 :2 + 3,7 ) : 0,2 = ( -3,1 + 3,7 ) : 0,2 = 0,6 : 0,2 = 3\)

\(9) \) \( 4 \dfrac{11}{23} - \dfrac{9}{14} + 2 \dfrac{12}{23} - \dfrac{5}{14}\)

\(= ( 4 \dfrac{11}{23} + 2 \dfrac{12}{23}) - ( \dfrac{5}{14} + \dfrac{9}{14} )\)

\(= 8- \dfrac{14}{14} \)

\(= 8-1\)

\(=7\)

\(10)\) \(11 \dfrac{1}{4} - ( 2 \dfrac{5}{7} + 5\dfrac{1}{4} )\)

\(= 11 \dfrac{1}{4} - 2 \dfrac{5}{7} - 5\dfrac{1}{4} \)

\(= 11 \dfrac{1}{4} - 5\dfrac{1}{4} - 2 \dfrac{5}{7}\)

\(= 6 - \dfrac{19}{7} = \dfrac{42}{7} - \dfrac{19}{7} = \dfrac{23}{7}\)

\(4,\dfrac{2}{7}\cdot\dfrac{3}{11}+\dfrac{2}{7}\cdot\dfrac{8}{11}=\dfrac{2}{7}\)

\(5,-12+\left(16-11\right)\cdot4=-12+20=8\)

\(6,2\dfrac{3}{7}+1\dfrac{4}{7}=\dfrac{17}{7}+\dfrac{11}{7}=\dfrac{28}{7}=4\)

\(7,\dfrac{-2}{3}\cdot\dfrac{4}{5}+\dfrac{1}{5}:\dfrac{9}{11}=\dfrac{-8}{15}+\dfrac{11}{45}=-\dfrac{13}{45}\)

\(\dfrac{2}{7}.\dfrac{3}{11}+\dfrac{2}{7}.\dfrac{8}{11}=\dfrac{2}{7}.\left(\dfrac{3}{11}+\dfrac{8}{11}\right)=\dfrac{2}{7}.1=\dfrac{2}{7}\)

\(-12+\left(16-11\right).4=-12+5.4=-12+20=8\)

\(2\dfrac{3}{7}+1\dfrac{4}{7}=\dfrac{17}{7}+\dfrac{11}{7}=\dfrac{28}{7}=4\)

\(\dfrac{-2}{3}.\dfrac{4}{5}+\dfrac{1}{5}:\dfrac{9}{11}=\dfrac{-8}{15}+\dfrac{11}{45}=\dfrac{-13}{45}\)

thiếu đề

a: Số học sinh giỏi là 40x1/5=8(bạn)

Số học sinh khá là 32x5/8=20(bạn)

SỐ học sinh trung bình là 32-20=12(bạn)

b: Tỉ số giữa số học sinh khá so với cả lớp là:

20:40=50%

8

\(40\cdot\dfrac{1}{5}=\dfrac{40}{5}=8\)

\(\dfrac{40}{1}\times\dfrac{1}{5}=\dfrac{40}{5}=8\)

8

= 8

8

8

8

8