F_{1} \cdot l_{1} - P_{1} \cdot \frac{l_{1}}{2} = F \cdot l_{2} - P_{2} \cdot \frac{l_{2}}{2}

${F_1\cdot l_1-P_1\cdot \frac{l_1}{2} = F\cdot l_2-P_2\cdot \frac{l_2}{2}}$

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