Bài 76: trang 39 sgk Toán 6 tập 2
Tính giá trị biểu thức sau một cách hợp lí:
\(A=\frac{7}{19}.\frac{8}{11}+\frac{7}{19}.\frac{3}{11}+\frac{12}{19}\)
\(B= \frac{5}{9}.\frac{7}{13}+\frac{5}{9}.\frac{9}{13}-\frac{5}{9}.\frac{3}{13}\)
\(C=\left (\frac{67}{111}+\frac{2}{33}-\frac{15}{117} \right ).\left (\frac{1}{3}-\frac{1}{4}-\frac{1}{12} \right )\)
Bài Làm:
\(A=\frac{7}{19}.\frac{8}{11}+\frac{7}{19}.\frac{3}{11}+\frac{12}{19}\)
\(=\frac{7}{19}.\left (\frac{8}{11}+\frac{3}{11} \right )+\frac{12}{19}\)
\(=\frac{7}{19}.1 +\frac{12}{19}=1\)
\(B= \frac{5}{9}.\frac{7}{13}+\frac{5}{9}.\frac{9}{13}-\frac{5}{9}.\frac{3}{13}\)
\(=\frac{5}{9}.\left (\frac{7}{13}+\frac{9}{13}-\frac{3}{13} \right )\)
\(=\frac{5}{9}.\frac{7+9-3}{13}\)
\(=\frac{5}{9}.\frac{13}{13}\)
\(=\frac{5}{9}.1=\frac{5}{9}\)
\(C=\left (\frac{67}{111}+\frac{2}{33}-\frac{15}{117} \right ).\left (\frac{1}{3}-\frac{1}{4}-\frac{1}{12} \right )\)
\(=\left (\frac{67}{111}+\frac{2}{33}-\frac{15}{117} \right ).\frac{4-3-1}{12}\)
\(=\left (\frac{67}{111}+\frac{2}{33}-\frac{15}{117} \right ).0=0\)