Answer
$7000$ lbs
Work Step by Step
Here, $A_i=2 \triangle x $
and $F_i=P_iA_i=62.5 \times x_i \times 2 \triangle x $
Thus, $F=\lim\limits_{n \to \infty} \Sigma_{i=1}^n 62.5(x_i) \times 2 dx $
or, $F=2 \times \int_3^{11} 62.5(x_i) \times 2 dx $
or, $F=62.5 \times [ x^2]_3^{11}$
or, $F=62.5(121-9)=7000$ lbs
