矩形の図心回りの断面2次moment

重心周りに計算する

https://kakeru.app/388cacfe0bd88c350fbe9f8e8e73c0f2 https://i.kakeru.app/388cacfe0bd88c350fbe9f8e8e73c0f2.svg

$ r_{xx}=\sqrt{\frac{I_{cxx}}{A}}=\sqrt{\frac{\frac1{12}a^3b}{ab}}=\frac1{2\sqrt3}a

$ r_{yy}=\sqrt{\frac{I_{cyy}}{A}}=\sqrt{\frac{\frac1{12}ab^3}{ab}}=\frac1{2\sqrt3}b