(-12) .(-8) > 90

a) (-12) .(-8) > 90

b) (-21) . (-3) > 59

c) (-15) .(-28) > 400

d) (-22) .(-21) > 420

a) (-4). (-8) > 0

b) (-23). 6 < (-4). (-15)

c) (+12) .(+7) < (-9). (-10)

(-12). 8 < (-19).3

(-123).8 < (-12).(-31)

 (-123).8 < (-12).(-31)

(-123).8 < (-12).(-31)

 (-123).8 = -984

 (-12).(-31) = 372

   mà -984 < 372 nên (-123).8 < (-12).(-31)

     HỌC TỐT NHA BẠN

Chú ý \(.\)là dấu nhân .

\(\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+...+\)\(\frac{1}{90}\)

=\(\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+...+_{\text{ }}\)\(\frac{1}{9.10}\)

= \(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\)\(\frac{1}{6}+...+\frac{1}{9}-\frac{1}{10}\)

= \(\frac{1}{2}-\frac{1}{10}\)

=\(\frac{2}{5}\)

Vì tử số bé hơn mẫu số nên phân số \(\frac{2}{5}\)< 1

A = \(\frac{3}{2}+\frac{7}{6}+\frac{13}{12}+...+\frac{91}{90}+\left(1+\frac{1}{2}\right)+\left(1+\frac{1}{6}\right)+\left(1+\frac{1}{12}\right)+...+\left(1+\frac{1}{90}\right)\)

\(=\left(1+\frac{1}{1.2}\right)+\left(1+\frac{1}{2.3}\right)+\left(1+\frac{1}{3.4}\right)+...+\left(1+\frac{1}{9.10}\right)\)

\(=\left(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{9.10}\right)+\left(1+1+1+...+1\right)\)(9 số hạng 1)

\(=\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{9}-\frac{1}{10}\right)+1.9\)

\(=\left(1-\frac{1}{10}\right)+9=10-\frac{1}{10}=\frac{99}{10}>\frac{98}{11}\)

a=1+1/1.2+1+1+1/2.3+....+1+1/9.10

a=1+1+...+1(9 chữ số 1)+1/1-1/2+1/2-1/3+...+1/9-1/10

a=9+1-1/10

a=9+9/10=9+0.9=9.9

b=98/11<98/10=9.8<9.9

=>vậy a>b