[Toán 8] Giải PT

N

nguyenbahiep1

[laTEX] (x^2+4x)(x^2+4x -5) = 84 \\ \\ u = x^2 + 4x \\ \\ u(u-5) - 84 = 0 \\ \\ u^2 -5u - 84 = 0 \\ \\ u = 12 \Rightarrow x^2 +4x - 12 = 0 \Rightarrow x = 2, x = - 6 \\ \\ u = - 7 \Rightarrow x^2 +4x +7 = 0 \Rightarrow (vo-nghiem)[/laTEX]
 
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