F.2 identities, 急!!!1天!!!

只要你知道 x^4 -y^4= ( x^2-y^2 ) ( x^2+y^2 )就行了

首先計算x^2-y^2

x^2-y^2

= √[(x^2+y^2)^2 - 4(xy)^2] <--------------因為 (x^2-y^2)^2 = (x^2+y^2) - 4(xy)^2

= √[(17)^2-4(7.5)^2]

= 8

所以x^4 -y^4

= ( x^2-y^2 ) ( x^2+y^2 )

= 8 x 17

=136

p.s. 用你個方法都得

(x+y)^2 = 32, (x-y)^2 = 2

兩條式乘埋,得到

[(x+y)^2] [(x-y)^2] = 64

(x^2-y^2)^2 = 64

x^2-y^2 = 8

之後一樣都計到x^4 -y^4=136

希望幫到你

2007-12-16 22:34:50 補充：
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x^4 -y^4
=(x^2+y^2)(x^2-y^2)
=17(x+y)(x-y)
=17√(32)√2
=17√(64)
=17*8
=136

我唔明^點解,我地冇教