Hướng dẫn trả lời:
a) `64 × 23 + 37 × 23 - 23.`
`= 64 × 23 + 37 × 23 - 1 × 23.`
`= (64 + 37 - 1) × 23.`
`= 100 × 23.`
`= 2300.`
b) `36 × 58 - 19 × 72 + 80.`
`= 36 × 58 - 19 × 2 × 36 + 80.`
`= 36 × 58 - 38 × 36 + 80.`
`= 36 × (58 - 38) + 80.`
`= 36 × 20 + 80.`
`= 36 × 20 + 4 × 20.`
`= (36 + 4) × 20.`
`= 40 × 20.`
`= 800.`
c)
Cách 1: `4 2/5 + 2 3/7 - 2 2/5 + 5 4/7.`
`= 22/5 + 17/7 - 12/5 + 39/7.`
`= (22/5 - 12/5) + (17/7 + 39/7).`
`= 10/5 + 56/7.`
`= 2 + 8.`
`= 10.`
Cách 2: `4 2/5 + 2 3/7 - 2 2/5 + 5 4/7.`
`= 4 + 2/5 + 2 + 3/7 - 2 - 2/5 + 5 + 4/7.`
`= (4 + 2 - 2 + 5) + (2/5 - 2/5) + (3/7 + 4/7).`
`= 9 + 7/7.`
`= 9 + 1.`
`= 10.`
d) `6 3/7 + 5 7/9 - (4 7/9 - 1 4/7).`
`= 6 3/7 + 5 7/9 - 4 7/9 + 1 4/7.`
Cách 1:
`= 6 + 3/7 + 5 + 7/9 - 4 - 7/9 + 1 + 4/7.`
`= (6 + 5 - 4 + 1) + (3/7 + 4/7) + (7/9 - 7/9).`
`= 8 + 7/7.`
`= 8 + 1.`
`= 9.`
Cách 2:
`= 45/7 + 52/9 - 43/9 + 11/7.`
`= (45/7 + 11/7) + (52/9 - 43/9).`
`= 56/7 + 9/9.`
`= 8 + 1.`
`= 9.`
e) $\dfrac{6×7×8×9×10×11}{22×20×18×16×14×12}.$
$=\dfrac{6×7×8×9×10×11}{11×2×10×2×9×2×8×2×7×2×6×2}.$
$=\dfrac{1}{2×2×2×2×2×2}.$
$=\dfrac{1}{2^6}.$
$=\dfrac{1}{64}.$
