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\(A=\frac{\left[\left(25-1\right):1+1\right]\left(25+1\right)}{2}=325.\)
\(B=\frac{\left[\left(51-3\right):2+1\right]\left(51+3\right)}{2}=675\)
\(C=\frac{\left[\left(81-1\right):4+1\right]\left(81+1\right)}{2}=861\)
a) \(\dfrac{3}{8}+\dfrac{15}{-25}+\dfrac{3}{5}\)
\(=\dfrac{-9}{40}+\dfrac{3}{5}\)
\(=\dfrac{3}{8}\)
b) \(\dfrac{-5}{18}+\dfrac{23}{45}-\dfrac{9}{10}\)
\(=\dfrac{7}{30}-\dfrac{9}{10}\)
\(=\dfrac{-2}{3}\)
c) \(\dfrac{-5}{12}+\dfrac{15}{18}-2,25\)
\(=\dfrac{5}{12}-2,25\)
\(=\dfrac{-11}{6}\)
d) \(\dfrac{5}{6}+\dfrac{2}{3}-0,5\)
\(=\dfrac{3}{2}-0,5\)
\(=1\)
Ta có: \(A=1-2+3-4+5-6+7-8+9\)
\(=(1+9)-(2+8)+(3+7)-(4+6)+5\)
\(=10-10+10-10+5\)
\(=5\)
Vậy \(A=5\)
B = 12 - 14 + 16 - 18 + ... + 2008 - 2010
B = -2 + (-2)+ (-2)+ (-2) + ...+ (-2)
B = -2 . 100
B = -200
5) 24*(15+49)+12*(50+42)
=24*64+12*92
=24*64+12*2*46
=24*64+24*46
=24*(64+46)
=24*110
=2640
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
-15 +40 = 40-15=25
-100-(-100) =100 -100=0
(-5).75+(-5).25=(-5).(75+25)= - 500
33 +2016o.(-5) = 27+1.(-5) = 22
tinh nhanh;
(-27)+42+18+27 = 2(8+9) =34
(-10-20).(-5) = (-30).(-5) =150
\(a,\dfrac{-3}{5}:\dfrac{15}{18}=\dfrac{-3}{5}\times\dfrac{18}{15}=\dfrac{-3}{5}\times\dfrac{6}{5}=\dfrac{-18}{25}\\ b,\dfrac{-5}{1}:\left(-4\right)=\left(-5\right):\left(-4\right)=\dfrac{5}{4}\\ c,-28:\dfrac{-7}{25}=-28\times\dfrac{25}{-7}=100\\ d,\dfrac{51}{8}:\dfrac{-2}{10}=\dfrac{51}{8}\times\dfrac{10}{-2}=\dfrac{51}{8}\times\left(-5\right)=\dfrac{-255}{8}\)
\(a)-3/5*18/15=-18/25\)
\(b)-5*1/-4=5/4\)
\(c)-28*25/-7=-100\)
\(d)51/8*10/-2=-255/8\)