Tìm x a) (x – 3)^3 – (x – 3) (x^2 + 3x + 9) + 9(x + 1)^2 =15 b) x(x – 5) (x + 5) – (x – 2) (x^2 + 2x +4) = -17 25/08/2021 Bởi Aubrey Tìm x a) (x – 3)^3 – (x – 3) (x^2 + 3x + 9) + 9(x + 1)^2 =15 b) x(x – 5) (x + 5) – (x – 2) (x^2 + 2x +4) = -17
a) (x – 3)3 – (x – 3)(x2 + 3x + 9) + 9(x + 1)2 = 15 => (x3 – 9x2 + 27x – 27) – (x3 – 27) + 9(x2 + 2x + 1) = 1 => x3 – 9x2 + 27x – 27 – x3 + 27 + 9x2 + 18x + 9 = 15 => 45x + 9 = 15 => 45x = 15 – 9 = 6 => x = $\frac{6}{45}$ => x = $\frac{2}{15}$ b) $x\left(x-5\right)\left(x+5\right)-\left(x-2\right)\left(x^2+2x+4\right)=-17$ $\Rightarrow x\left(x^2-25\right)-\left(x^3-8\right)=-17$ $\Rightarrow x^3-25x-x^3+8=-17$ => -25x = -17 – 8 = -25 => x = -25 : (-25) = 1 Bình luận
Đáp án:
Giải thích các bước giải:
a) (x – 3)3 – (x – 3)(x2 + 3x + 9) + 9(x + 1)2 = 15
=> (x3 – 9x2 + 27x – 27) – (x3 – 27) + 9(x2 + 2x + 1) = 1
=> x3 – 9x2 + 27x – 27 – x3 + 27 + 9x2 + 18x + 9 = 15
=> 45x + 9 = 15
=> 45x = 15 – 9 = 6
=> x = $\frac{6}{45}$
=> x = $\frac{2}{15}$
b) $x\left(x-5\right)\left(x+5\right)-\left(x-2\right)\left(x^2+2x+4\right)=-17$
$\Rightarrow x\left(x^2-25\right)-\left(x^3-8\right)=-17$
$\Rightarrow x^3-25x-x^3+8=-17$
=> -25x = -17 – 8 = -25
=> x = -25 : (-25) = 1