Tính bằng 2 cách. 54:6-42:6
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48 : 6 = 8 42 : 6 = 7 6 : 6 = 1 18 : 6 = 3
24 : 6 = 4 36 : 6 = 6 30 : 6 = 5 18 : 3 = 6
a, 42x(54+17):71
=42x71:71=42
b, 123x(154-65):89
=123x89:89=123
c,324x(6+4):324
=324x10:324=10x324:324=10
\(a)\left(42\times54+17\times42\right):71\)
\(=42\times54+17\times42:71\)
\(=42\times\left(54+17\right):71\)
\(=42\times71:71\)
\(=42\)
\(b)\left(123\times154-65\times123\right):89\)
\(=123\times154-65\times123:89\)
\(=123\times\left(154-65\right):89\)
\(=123\times89:89\)
\(=123\)
\(c)\left(324\times6+4\times324\right):\left(162\times2\right)\)
\(=324\times6+4\times324:324\)
\(=324\times\left(6+4\right):324\)
\(=324\times10:324\)
\(=3240:324\)
\(=10\)
Học sinh nhẩm và ghi:
6 x 6 = 36
6 x 9 = 54
6 x 7 = 42
6 x 8 = 48
36 : 6 = 6
54 : 6 = 9
42 : 6 = 7
48 : 6 = 8
6 x 6 = 36 7 x 9 = 63 8 x 7 = 56 9 x 6 = 54
48 : 6 = 8 54 : 6 = 9 42 : 6 = 42 63 : 7 = 9
6 x 6 = 36 7 x 9 = 63 8 x 7 = 56 9 x 6 = 54
48 : 6 = 8 54 : 6 = 9 42 : 6 = 42 63 : 7 = 9
\(\dfrac{1}{2}+\dfrac{1}{6}+\dfrac{1}{12}+\dfrac{1}{20}+\dfrac{1}{30}+\dfrac{1}{42}\)
\(=\dfrac{1}{1\cdot2}+\dfrac{1}{2\cdot3}+\dfrac{1}{3\cdot4}+\dfrac{1}{4\cdot5}+\dfrac{1}{5\cdot6}+\dfrac{1}{6\cdot7}\)
\(=1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}\)
\(=1-\dfrac{1}{7}\)
\(=\dfrac{6}{7}\)
\(\dfrac{1}{2}+\dfrac{1}{6}+\dfrac{1}{12}+\dfrac{1}{20}+\dfrac{1}{30}+\dfrac{1}{42}\)
\(=\dfrac{1}{1\cdot2}+\dfrac{1}{2\cdot3}+\dfrac{1}{3\cdot4}+\dfrac{1}{4\cdot5}+\dfrac{1}{5\cdot6}+\dfrac{1}{6\cdot7}\)
\(=1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}\)
\(=1-\dfrac{1}{7}\)
\(=\dfrac{6}{7}\)
42 : 6 = 7 48 : 6 = 8 6 : 6 = 1 12 : 3 = 4
18 : 6 = 3 54 : 6 = 9 30 : 6 = 5 12 : 4 = 3
C1: 54 : 6 - 42 : 6
= (54 - 42) : 6
= 12 : 6
= 2
C2: 54 : 6 - 42 : 6
= (54 : 6) - (42 : 6)
= 9 - 7
= 2