Đáp án:
Bài `8`
`a, A= (a + b + c)^2 = [(a + b) + c]^2`
`= (a + b)^2 + 2(a + b).c + c^2`
`= a^2 + 2ab + b^2 + 2ac + 2bc + c^2`
`= a^2 + b^2 + c^2 + 2ab + 2ac + 2bc`
`b, B = (a - b - c)^2 = [(a - b) - c]^2`
`= (a - b)^2 - 2(a - b)c + c^2`
`= a^2 - 2ab + b^2 - (2a - 2b).c + c^2`
`= a^2 - 2ab + b^2 - 2ac + 2bc + c^2`
`= a^2 + b^2 + c^2 - 2ab - 2ac + 2bc`
Bài `9`
`a, 199^2 = (200 - 1)^2 = 200^2 - 2.200.1 + 1^2`
`= 40 000 - 400 + 1 = 39 601`
`b, 99^2 = (100 - 1)^2 = 100^2 - 2.100.1 + 1^2`
`= 10 000 - 200 +1 = 9801`
`c, 499^2 = (500 -1 )^2 = 500^2 - 2.500.1 + 1^2`
`= 250 000 - 1000 + 1`
`= 249 001`
`d, 299.301 = (300 - 1)(300 + 1) = 300^2 - 1^2 = 90 000 - 1= 89 999`
