### 2 Answers

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- AshLv 71 month ago
A) Moment of Inertia of outer box ...

Iox = a³(b + 30 + 30)/12 = (230³)(280 + 30 + 30)/12 = 230³(85)/3

Moment of Inertia of hollow area ...

Iix = (a-30-30)³b/12 = (230-30-30)³(280)/12 = 170³(70)/3

Moment of inertia of hollow box = Iox - Iix

= 230³(85)/3 - 170³(70)/3

= [230³(85) - 170³(70)]/3

= 230,095,000 mm⁴

= 2.30 x 10⁸ mm⁴ ← answer to 3 sig fig

B) Moment of Inertia of outer box ...

Ioy = a(b + 30 + 30)³/12 = (230)(280 + 30 + 30)³/12 = 115(340)³/6

Moment of Inertia of hollow area ...

Iiy = (a-30-30)b³/12 = (230-30-30)(280)³/12 = 85(280)³/6

Moment of inertia of hollow box = Ioy - Iiy

= 115(340)³/6 - 85(280)³/6

= [115(340)³) - 85(280)³]/6

= 442,340,000 mm⁴

= 4.42 x 10⁸ mm⁴ ← answer to 3 sig fig

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