计算:(x+2y)(x-2y)(x^2-4y^2),其中x=5/3,y=2/3.

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计算:(x+2y)(x-2y)(x^2-4y^2),其中x=5/3,y=2/3.
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计算:(x+2y)(x-2y)(x^2-4y^2),其中x=5/3,y=2/3.
计算:(x+2y)(x-2y)(x^2-4y^2),其中x=5/3,y=2/3.

计算:(x+2y)(x-2y)(x^2-4y^2),其中x=5/3,y=2/3.
=(x^2-4y^2)(x^2-4y^2)
=(x^2-4y^2)^2
=[(5/3)^2-4*(2/3)^2]^2
=(25/9-4*4/9)^2
=(25/9-16/9)^2
=1

原式=(x+2y)(x-2y)(x+2y)(x-2y)
=(x+2y)^2(x-2y)^2
=9*1/9=1

(x2+4y2)(x2-4y2)=(x2)2-(4y2)2=x4-16y4
代入x=5/3,y=2/3.得(5/3)4-16*(2/3)4=625/81-16*(18/81)=625/81-288/81=337/81=4*13/81=4.16