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a) Ta có: \(\dfrac{-5}{7}\left(\dfrac{14}{5}-\dfrac{7}{10}\right):\left|-\dfrac{2}{3}\right|-\dfrac{3}{4}\left(\dfrac{8}{9}+\dfrac{16}{3}\right)+\dfrac{10}{3}\left(\dfrac{1}{3}+\dfrac{1}{5}\right)\)
\(=\dfrac{-5}{7}\cdot\dfrac{3}{2}\cdot\dfrac{21}{10}-\dfrac{3}{4}\cdot\dfrac{56}{3}+\dfrac{10}{3}\cdot\dfrac{8}{15}\)
\(=\dfrac{-9}{4}-14+\dfrac{16}{9}\)
\(=\dfrac{-1621}{126}\)
b) Ta có: \(\dfrac{17}{-26}\cdot\left(\dfrac{1}{6}-\dfrac{5}{3}\right):\dfrac{17}{13}-\dfrac{20}{3}\left(\dfrac{2}{5}-\dfrac{1}{4}\right)+\dfrac{2}{3}\left(\dfrac{6}{5}-\dfrac{9}{2}\right)\)
\(=\dfrac{-17}{26}\cdot\dfrac{13}{17}\cdot\dfrac{-3}{2}-\dfrac{20}{3}\cdot\dfrac{3}{20}+\dfrac{2}{3}\cdot\dfrac{-33}{10}\)
\(=\dfrac{3}{4}-1-\dfrac{11}{5}\)
\(=-\dfrac{49}{20}\)
nhóm cứ hai số liên tiếp lại vs nhâu sẽ đk quy luật tổng của toàn số -2
A=2-4+6-8+10-12+14-16+18-20+22-24+26-28+30-32
A= (-2)+(-2)+(-2)+(-2)+(-2)+(-2)+(-2)+(-2)
A= -16
a) n(n+2)
b) (3n-2)3n
c) ( 1) 1 n n 2
d) 1+n2 e) n(n+5)
f) (3n-2)(3n+1)
g) n ( n 3) 2 n n
h) ( 1)( 2) 2
i) n ( n 1)( n 2)
1+2+3+4+5+6+7+8+9+10=55
11+12+13+14+15+16+17+18+19+20=155
1+2+3+4+5+6+7+8+9+10+11+12+13+14 +15+16+17+18+19+20+21+22+23+24+25+26+27+28+29+30-50-53=362
1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20+21+22+23+24+25+26+27+28+29+30 =465
\(D=\frac{3}{2.6}+\frac{3}{6.10}+\frac{3}{10.14}+..+\frac{3}{26.30}\)
\(=\frac{3}{4}.\left(\frac{4}{2.6}+\frac{4}{6.10}+\frac{4}{10.14}+...+\frac{4}{26.30}\right)\)
\(=\frac{3}{4}\left(\frac{1}{2}-\frac{1}{6}+\frac{1}{6}-\frac{1}{10}+\frac{1}{10}-\frac{1}{14}+...+\frac{1}{26}-\frac{1}{30}\right)\)
\(=\frac{3}{4}\left(\frac{1}{2}-\frac{1}{30}\right)=\frac{3}{4}.\frac{7}{15}=\frac{7}{20}\)
\(D=\frac{3}{2.6}+\frac{3}{6.10}+..+\frac{3}{26.30}\)
\(D=3\left(\frac{1}{2.6}+\frac{1}{6.10}+..+\frac{1}{26.30}\right)\)
\(4D=4.3.\left(\frac{1}{2.6}+\frac{1}{6.10}+..+\frac{1}{26.30}\right)\)
\(4D=3\left(\frac{4}{2.6}+\frac{4}{6.10}+..+\frac{4}{26.30}\right)\)
\(4D=3\left(\frac{1}{2}-\frac{1}{6}+\frac{1}{6}-\frac{1}{10}+..+\frac{1}{26}-\frac{1}{30}\right)\)
\(4D=3\left(\frac{1}{2}-\frac{1}{30}\right)\)
\(4D=3.\frac{7}{15}\)
\(4D=\frac{7}{5}\)
\(D=\frac{7}{5}:4=\frac{7}{20}\)
\(\text{Vậy D =}\frac{7}{20}\)