[toán 8] tính giá trị biểu thức

T

transformers123

a/ $8x(2x-1)-(4x-1)^2-13$

$=16x^2-8x-16x^2+8x-1-13$

$=-14$

Kết luận ....

b/ $(x+2)(x^2-2x+4)=\dfrac{91}{8}$

$\iff x^3+8=\dfrac{91}{8}$

$\iff x^3=\dfrac{27}{8}$

$\iff x=\dfrac{3}{2}$

Kết luận ....
 
T

thangvegeta1604

1) $8x(2x-1)-(4x-1)^2-13$
=$16x^2-8x-(16x^2-8x+1)-13$
=$16x^2-8x-16x^2+8x-1-13=-14$.
2) $(x+2)(x^2-2x+4)=\dfrac{91}{8}$
\Rightarrow $x^3+2^3=\dfrac{91}{8}$
\Rightarrow $x^3=\dfrac{27}{8}$
\Rightarrow $x=\dfrac{3}{2}$
 
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