Câu hỏi 21Khai triển $\left(3x-\dfrac12\right)^3$.$27x^3-\frac18$$27x^3-\frac{27}2x^2-\frac94x+\frac18$$27x^3-\frac{27}2x^2+\frac94x-\frac18$$27x^3-\frac92x^2+\frac92x-\frac18$$9x^3-\frac9x x^2+\frac94x-\frac18$