Đáp án:
`1d)`
` (6ax+7)^2`
` = (6ax)^2 + 2.6ax.7 + 7^2`
` = 36a^2x^2 + 84ax + 49`
` e)`
` (a+b-c)^2`
` = a^2 + b^2 + c^2 + 2.a.b + 2.b.(-c) + 2.a.(-c)`
` = a^2 + b^2 + c^2 + 2ab - 2bc - 2ac`
`f)`
` (a+2b +3c)^2`
` = a^2+ (2b)^2 + (3c)^2 + 2.a.2b + 2.(2b).(3c) + 2.a.(3c)`
` = a^2 + 4b^2 + 9c^2 + 4ab + 12bc + 6ac`