F2 expanding 1題only!!!

2009-10-01 10:55 pm

(2x+7)^2-(7-2x)^2
本書話答案係56x，但我計唔到= =

回答 (5)

2009-10-01 11:46 pm

✔ 最佳答案

(2x+7)^2-(7-2x)^2
=(2x+7)^2-(2x-7)^2 <--- (y-x)^2 = (x-y)^2
=[(2x+7)+(2x-7)][(2x+7)-(2x-7)] <--- x^2 - y^2 = (x+y)(x-y)
=(4x)(7+7)
=(4)(14)(x)
=56x

參考: me

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2009-10-02 6:00 am

a^2-b^2=(a+b)(a-b)

(2x+7)^2-(7-2x)^2
=(2x+7+7-2x)(2x+7-7+2x)
=14(4x)
=56x

參考: myself

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2009-10-02 4:42 am

By Using Identities Or Not??

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2009-10-02 2:03 am

[(2x+7)-(7-2x)] [(2x+7)+(7-2x)]
=( 4x)(14)
=56x

參考: 自己

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2009-10-01 10:56 pm

(2x+7)^2-(7-2x)^2
=(4x^2+28x+49)-(49-28x+4x^2)
=4x^2+28x+49-49+28x-4x^2
=56x

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