For section 1, we have $A_1 = 2 \times (50 + 20) = 140 \, \text{mm}^2$. For section 2, we have $A_2 = 75 \times 20 = 1500 \, \text{mm}^2$. Since the cross-section of section 2 is the same as section 3, we have $A_3 = A_2 = 1500 \, \text{mm}^2$.
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