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729=\int_{0}^{\mathrm{T}} 3 t^{2} \mathrm{dt}=\left.t^{3}\right|_{0} ^{\mathrm{T}}=(\mathrm{T})^{3}-(0)^{3}=\mathrm{T}^{3}, \text { so } \mathrm{T}=\sqrt[3]{729}=9