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Câu 9
a) 2/15 + (-3/5)
= 2/15 - 3/5
= 2/15 - 9/15
= -7/15
b) -5/7 + 3/7 . (-2/9)
= -5/7 - 2/21
= -15/21 - 2/21
= -17/21
c) 5/16 - (7/15 - 3/16) + 17/30
= 5/16 - 7/15 + 3/16 + 17/30
= (5/16 + 3/16) + (17/30 - 14/30)
= 1/2 + 1/10
= 5/10 + 1/10
= 6/10
= 3/5
\(a,\dfrac{2}{15}+\left(-\dfrac{3}{5}\right)\\ =\dfrac{2}{15}-\dfrac{3}{5}\\ =\dfrac{2}{15}-\dfrac{9}{15}\\ =-\dfrac{7}{15}\\ b,-\dfrac{5}{7}+\dfrac{3}{7}\times\left(-\dfrac{2}{9}\right)\\ =-\dfrac{5}{7}-\dfrac{2}{21}\\ =-\dfrac{15}{21}-\dfrac{2}{21}\\ =-\dfrac{17}{21}\\ c,\dfrac{5}{16}-\left(\dfrac{7}{15}-\dfrac{3}{16}\right)+\dfrac{17}{30}\\ =\dfrac{5}{16}-\dfrac{7}{15}+\dfrac{3}{16}+\dfrac{17}{30}\\ =\left(\dfrac{5}{16}+\dfrac{3}{16}\right)-\left(\dfrac{17}{15}+\dfrac{17}{30}\right)\\ =\dfrac{1}{2}-\dfrac{17}{10}\\ =\dfrac{5}{10}-\dfrac{17}{10}\\ =-\dfrac{6}{5}.\)
a ) 14
b) giá trị tuyệt đối thuộc : 1,2,3,4,5,6,7
x thuộc -1,1,-2,2,-3,3........-7,7
tổng =0
a) = ( 9/10 - 4/5) + ( 3/4 + 1/2)
= ( 9/10 - 8/10) + ( 3/4 + 2/4)
= 1/10 + 5/4
= 2/20 + 25/20 = 27/20
b) = (4/5 + 4/15) + (5/6 - 1/2)
= (12/15 - 4/15) + (5/6 - 3/6)
= 8/15 + 2/6
= 16/30 + 10/30
= 26/30 = 13/15
c) = 55 - 39 - 1 + 60 /75 = 75/75 = 1
d) = 47 + 35 - 36 + 15/42 = 61/42
a: \(\dfrac{1}{1\cdot2}+\dfrac{1}{2\cdot3}+...+\dfrac{1}{100\cdot101}\)
=1-1/2+1/2-1/3+...+1/100-1/101
=1-1/101=100/101
b: \(A=1+\dfrac{1}{2}+1+\dfrac{1}{6}+1+\dfrac{1}{12}+...+1+\dfrac{1}{10100}\)
\(=100+\left(1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+...+\dfrac{1}{100}-\dfrac{1}{101}\right)\)
\(=101-\dfrac{1}{101}< 101\)
\(\frac{7}{8}\div\left(\frac{14}{3}+\frac{14}{4}\right)+\frac{10}{14}\)
\(=\frac{7}{8}\div\left(\frac{56}{12}+\frac{52}{12}\right)+\frac{10}{14}\)
\(=\frac{7}{8}\div\frac{108}{12}+\frac{10}{14}\)
\(=\frac{7}{8}\div9+\frac{5}{7}\)
\(=\frac{7}{8}\times\frac{1}{9}+\frac{5}{7}\)
\(=\frac{7}{72}+\frac{5}{7}\)
\(=\frac{49}{504}+\frac{360}{504}\)
\(=\frac{409}{504}\)
\(\frac{30}{42}\div\left(\frac{5}{7}-\frac{5}{21}\right)+\frac{6}{8}\)
\(=\frac{6}{7}\div\left(\frac{15}{21}-\frac{5}{21}\right)+\frac{3}{4}\)
\(=\frac{6}{7}\div\frac{10}{21}+\frac{3}{4}\)
\(=\frac{6}{7}\times\frac{21}{10}+\frac{3}{4}\)
\(=\frac{126}{70}+\frac{3}{4}\)
\(=\frac{504}{280}+\frac{210}{280}\)
\(=\frac{714}{280}\)
\(\left(6-\frac{14}{15}\right).\frac{50}{38}-\frac{8}{18}\)
\(=\left(\frac{90}{15}-\frac{14}{15}\right).\frac{25}{19}-\frac{4}{9}\)
\(=\frac{76}{15}.\frac{25}{19}-\frac{4}{9}\)
\(=\frac{76.25}{15.19}-\frac{4}{9}\)
\(=\frac{19.2.2.5.5}{3.5.19}-\frac{4}{9}\)
\(=\frac{20}{3}-\frac{4}{9}\)
\(=\frac{60}{9}-\frac{4}{9}\)
\(=\frac{56}{9}\)
\(\frac{4}{6}+\frac{5}{21}.\left(\frac{4}{5}-\frac{2}{6}\right)\)
\(=\frac{2}{3}+\frac{5}{21}.\left(\frac{24}{30}-\frac{10}{30}\right)\)
\(=\frac{2}{3}+\frac{5}{21}.\frac{14}{30}\)
\(=\frac{2}{3}+\frac{70}{630}\)
\(=\frac{420}{630}+\frac{70}{630}\)
\(=\frac{490}{630}\)
\(=\frac{7}{9}\)
\(a,\dfrac{5}{7}-\dfrac{11}{4}\)
\(=\dfrac{20}{28}-\dfrac{77}{28}\)
\(=\dfrac{20-77}{28}\)
\(=-\dfrac{57}{28}\)
\(b,\dfrac{15}{7}:-\dfrac{18}{14}\)
\(=\dfrac{15}{7}\times\left(-\dfrac{14}{18}\right)\)
\(=\dfrac{15\times\left(-14\right)}{7\times18}=-\dfrac{210}{126}=-\dfrac{5}{3}\)