※ 引述《pinkemily (沒大腦女孩)》之銘言：
: 剛算工數算到聯立線性O.D.E
: (t^2D^2+tD)(t^2D^2)=t^4D^4+5t^3D^3+4t^2D^2
: 想問一下中間過程是怎麼算的...
2 2 2 2 2 2 2 2 2 2
(t D + tD)(t D ) = (t D )(t D ) + tD(t D )
2 2 2 3 2 2 3
= (t D)(2tD + t D ) + t(2tD + t D )
2 2 3 3 2 4 2 2 3 3
= t (2D + 2tD + 2tD + t D ) + 2t D + t D
4 4 3 3 2 2
= t D + 5t D + 4t D
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