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a) $~(x+3)^2-x^2+5x-9$ $= x^2 + 6x + 9 - x^2 + 5x - 9$ $= (x^2 - x^2) + (6x + 5x) + (9 - 9)$ $= 11x$ b) $~(x+4)^2-(x-4)^2$ $= (x^2 + 8x + 16) - (x^2 - 8x + 16)$ $= x^2 + 8x + 16 - x^2 + 8x - 16$ $= (x^2 - x^2) + (8x + 8x) + (16 - 16)$ $= 16x$ c) $~x(x-5)+(x-5)^2-2x^2+15$ $= x^2 - 5x + (x^2 - 10x + 25) - 2x^2 + 15$ $= x^2 - 5x + x^2 - 10x + 25 - 2x^2 + 15$ $= (x^2 + x^2 - 2x^2) + (-5x - 10x) + (25 + 15)$ $= 0 - 15x + 40$ $= -15x + 40$ Vậy, các biểu thức đã rút gọn như sau: a) $11x$ b) $16x$ c) $-15x + 40$