Bài 1: Tính
a) |-3|.1/2-3².3/2
b) A= 1/1.2+1/2.3+1/3.4+…….+1/15.16
c) 3 mũ 4.3³-1/4.5/2-1
Bài 2: Tìm xEz
a) x/-14=2/7
b) x+1/3=7/5
Bài 1: Tính a) |-3|.1/2-3².3/2 b) A= 1/1.2+1/2.3+1/3.4+…….+1/15.16 c) 3 mũ 4.3³-1/4.5/2-1 Bài 2: Tìm xEz a) x/-14=2/7 b) x+1/3=7/5
By Reese
Bài 1
a) l$-3$l . $\frac{1}{2} – 3^2 . \frac{3}{2} $
= 3 . $\frac{1}{2} – 9. \frac{3}{2} $
= $\frac{3}{2} – \frac{27}{2} = -12$
b) $\frac{1}{1.2} + \frac{1}{2.3} + … + \frac{1}{15.16}$
= $( 1 – \frac{1}{2} ) + (\frac{1}{2} – \frac{1}{3} ) +… + (\frac{1}{15}- \frac{1}{16} )$
= $1 – (\frac{1}{2} – \frac{1}{2} ) – (\frac{1}{3} + \frac{1}{3} ) -….. – (\frac{1}{15} – \frac{1}{15}) – \frac{1}{16}$
= $ 1 – \frac{1}{16} $
= $\frac{15}{16}$
Bài 2
a) $\frac{x}{-14} = \frac{2}{7}$
=> $\frac{x}{-14} = \frac{-4}{-14}$
=> $x = -4$
b) $ x + \frac{1}{3} = \frac{7}{5}$
=> $ x = \frac{7}{5} – \frac{1}{3} $
=> $ x = \frac{21}{15} – \frac{5}{15} $
=> $x = \frac{16}{15}$
Đáp án:
Bài 1:
$a) -12\\
b) A=\frac{15}{16}\\
c) \frac{17483}{8}$
Bài 2:
$a) x=-4\\
b) x=\frac{16}{15}$
Giải thích các bước giải:
Bài 1:
$a) |-3|.\frac{1}{2}-3^2.\frac{3}{2}\\
=3.\frac{1}{2}-9.\frac{3}{2}\\
=\frac{3}{2}-9.\frac{3}{2}\\
=\frac{3}{2}(1-9)\\
=-8.\frac{3}{2}\\
=\frac{-24}{2}\\
=-12\\
b) A= \frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+…….+\frac{1}{15.16}\\
=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+….+\frac{1}{15}-\frac{1}{16}\\
=1-\frac{1}{16}\\
=\frac{16}{16}-\frac{1}{16}\\
=\frac{15}{16}\\
c) 3^4.3^3-\frac{1}{4}.\frac{5}{2}-1\\
=3^7-\frac{5}{8}-1\\
=2187-\frac{5}{8}-\frac{8}{8}\\
=\frac{2187.8}{8}-\frac{13}{8}\\
=\frac{17483}{8}$
Bài 2:
$a) \frac{x}{-14}=\frac{2}{7}\\
\Leftrightarrow x=\frac{2.(-14)}{7}\\
\Leftrightarrow x=-4\\
b) x+\frac{1}{3}=\frac{7}{5}\\
\Leftrightarrow x=\frac{7}{5}-\frac{1}{3}\\
\Leftrightarrow x=\frac{21}{15}-\frac{5}{15}\\
\Leftrightarrow x=\frac{16}{15}$