Đáp án:
Giải thích các bước giải:
a) `(x + 7)^2 + (x - 2)^2 - 2(x + 7)(x - 2)`
`= (x + 7)^2 - 2(x + 7)(x - 2) + (x - 2)^2`
`= [(x + 7) - (x - 2)]^2`
`= (x + 7 - x + 2)^2`
`= 9^2 = 81`
b) `3(x - 3)(x + 7) + (x - 4)^2 + 48`
`= 3(x^2 + 7x - 3x - 21) + x^2 - 8x + 16 + 48`
`= 3x^2 + 21x - 9x - 63 + x^2 - 8x + 16 + 48`
`= (3x^2 + x^2) + (21x - 9x - 8x) + (-63 + 16 + 48)`
`= 4x^2 + 4x + 1`
`= (2x + 1)^2`
c) `(x + y)^2 - (x - y)^2`
`= [(x + y) - (x - y)][(x + y) + (x - y)]`
`= (x + y - x + y)(x + y + x - y)`
`= 2y . 2x = 4xy`
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