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a) \(\dfrac{-15+9+11}{16}=\dfrac{5}{16}\)
b) \(\dfrac{2}{3}\left(1,4+1,6-1,2\right)=\dfrac{2}{3}\times\dfrac{9}{5}=\dfrac{6}{5}\)
c) \(3\dfrac{2}{15}\left(\dfrac{3}{5}+\dfrac{2}{5}\right)-\dfrac{31}{15}=\dfrac{47}{15}-\dfrac{31}{15}=\dfrac{16}{15}\)
Đặt tổng trên = A
\(A=\frac{15}{1.2}+\frac{15}{2.3}+\frac{15}{3.4}+...+\frac{15}{19.20}\)
\(A:15=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{19.20}\)
\(A:15=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{19}-\frac{1}{20}\)
\(A:15=1-\frac{1}{20}=\frac{19}{20}\)
\(A=\frac{19}{20}.15=\frac{57}{4}\)
\(A=\frac{15}{1.2}+\frac{15}{2.3}+\frac{15}{3.4}+...+\frac{15}{19.20}\)
\(A:15=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{19.20}\)
\(A:15=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{19}-\frac{1}{20}\)
\(A:15=1-\frac{1}{20}=\frac{19}{20}\)
\(A=\frac{19}{20}.15=\frac{57}{4}\)
1. \(\dfrac{-1}{7}-\dfrac{1}{8}=\dfrac{-8-7}{56}=\dfrac{-15}{56}.\)
2. \(\dfrac{15}{48}-\dfrac{5}{12}=\dfrac{5}{16}-\dfrac{5}{12}=-\dfrac{5}{48}.\)
3. \(\dfrac{-3}{15}-\left(\dfrac{-3}{25}\right)=\dfrac{-1}{5}+\dfrac{3}{25}=\dfrac{-2}{25}.\)
4. \(\dfrac{1}{15}-\dfrac{3}{5}=\dfrac{1-9}{15}=-\dfrac{8}{15}.\)
1. \(\dfrac{-1}{7}-\dfrac{1}{8}=\dfrac{-8-7}{56}=-\dfrac{15}{56}.\)
2. \(\dfrac{15}{48}-\dfrac{5}{12}=\dfrac{5}{16}-\dfrac{5}{12}=-\dfrac{5}{48}.\)
3. \(\dfrac{-3}{15}-\left(-\dfrac{3}{25}\right)=\dfrac{-1}{5}+\dfrac{3}{25}=-\dfrac{2}{25}.\)
4. \(\dfrac{1}{15}-\dfrac{3}{5}=\dfrac{1-9}{15}=-\dfrac{8}{15}.\)
\(\left(15+x\right):3=3^{15}:3^{12}\)
\(\left(15+x\right)=3^3.3\)
\(15+x=3^4\)
\(x=81-15\)
\(x=66\)
\(\left(15+x\right):3=3^{15}:3^{12}\)
\(\left(15+x\right):3=3^{15-12}\)
\(\left(15+x\right):3=3^3\)
\(15+x=3^3.3\)
\(15+x=81\)
\(x=81-15\)
\(x=66\)