Đáp án:
`a)` `18225.`
`b)` `3600000.`
Giải thích các bước giải:
`a)` `2021+2022+2023+2024+2025+2026+2027+2028+2029.`
`= (2021+2029)+(2022+2028)+(2023+2027)+(2024+2026)+2025.`
`= 4050+4050+4050+4050+2025.`
`= 4050\times4+2025.`
`= 16200+2025.`
`=18225.`
`b)` `30\times40\times50\times60.`
`= (3\times10)\times(4\times10)\times(5\times10)\times(6\times10).`
`= 3\times10\times4\times10\times5\times10\times6\times10.`
`= (3\times4)\times(5\times6)\times(10\times10\times10\times10).`
`= 12\times30\times10000.`
`= 3600000.`
