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\int_{0}^{\mathrm{T}} 6 t \mathrm{dt}=\left.3 t^{2}\right|_{0} ^{\mathrm{T}}=3(\mathrm{~T})^{2}-3(0)^{2}=3 \mathrm{~T}^{2} \text { so } \mathrm{T}^{2}=400