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\int_{0}^{30}(70-2 x) \mathrm{dx}=70 x-\left.x^{2}\right|_{0} ^{30}=\left\{70 \cdot 30-(30)^{2}\right\}-\left\{70 \cdot 0-(0)^{2}\right\}=(2100-900)-(0)=1200 foot–pounds