Ix, Iy, Ixy. y x1, y1.
.
X1= x-a; y1=y-b
I x1= ∫ y1 dA = ∫ (y-b)2 dA = ∫ (y2- 2by + b3)dA = ∫ y2 dA 2b ∫ ydA + b2 ∫dA=
=Ix 2b Sx + b2A.
x , Sx =0.
I x1= Ix + b2A
y1 I y1= Iy + a2A
Ix1y1= Ixy b Sx a Sy + abA.
xy , Ix1y1= Ixy + abA
, , Ixy =0, Ix1y1= abA
.
xy.
xy (a >0), .
y1 =ab = ac bc = ab- de
acd:
ac/ad =cos α ac= ad*cos α
oed:
de/od =sin α dc = od*sin α
y
y1 = ad cos α - od sin α = y cos α - x sin α.
x1 = x cos α + y sin α.
x1
Ix1 = ∫y12 dA = ∫ (y cos α - x sin α)2dA= ∫ (y2 cos2 α - 2xy sin α cos α + x2 sin2 α)dA= =cos2 α ∫ y2 dA sin2 α ∫xy dA + sin2 α ∫x2 dA = Ix cos2 α - Ixy sin2 α + Iy sin2 α.
Iy1 = Ix sin2 α - Ixy sin2 α + Iy cos2 α .
:
Ix1 + Iy1 = Ix (sin2 α + cos2 α) + Iy (sin2 α + cos2 α) + Ixy (sin2 α - cos2 α).
:
Ix1 + Iy1 = Ix + Iy
.
. x1 ,y1 .
Ix1y1 = ∫x1 y1 dA = (Ix Iy)/2*sin 2 α + Ixy cos 2 α.
.
.
, . .
0. , , 0, . I x1 , 0, = .
tg2 α0 = -
α0 >0, . max, min. max , . :
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