查看完整版本 : 用配方法解二次方程既計法想大家幫幫手

火雲雷神 2017-5-31 12:32 AM

用配方法解二次方程既計法想大家幫幫手

x2 + 9x +  20.25 = (x + 4.5 )^2

我想知點解會係4.5 因為我計左係10.125

x2 - 1/2x + 0.0625 = (x + -0.25)^2

點解會係-0.025 我計到係 0.03125

x2 + 4px + 4p2 = (x+ 2p)^2

呢題我唔知4px 同 2p 係點樣計出來?

edok 2017-6-10 10:50 PM

[quote]原帖由 [i]火雲雷神[/i] 於 2017-5-31 12:32 AM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=462166619&ptid=26689692][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]
x2 + 9x +  20.25 = (x + 4.5 )^2

我想知點解會係4.5 因為我計左係10.125

x2 - 1/2x + 0.0625 = (x + -0.25)^2

點解會係-0.025 我計到係 0.03125

x2 + 4px + 4p2 = (x+ 2p)^2

呢題我唔知4px 同 2p 係點樣計出來? [/quote]



.        以 a^2 + 2ab + b^2 = (a + b)^2  為例

你題目係 x^2 + 9x + 20.25 , 你依家未必完全明白個公式或者完全唔明白做緊乜,

但你最緊要係要懂得公式對位...你對到個位, 你就識代入公式

我依家將你條題目, 整齊對正條公式....

你就會清清楚楚見到, 你中間個 "9" 係要靠某個數乘上 "2" ,

然後你再試下, 4.5^2 係咪等於 20.25 , 咁你就會得出 x^2 + 9x + 20.25 = (x + 4.5)^2

後面2題, 同樣做法

[[i] 本帖最後由 edok 於 2017-6-10 10:51 PM 編輯 [/i]]

chanrichard 2017-7-30 12:48 PM

[quote]原帖由 [i]火雲雷神[/i] 於 2017-5-31 12:32 AM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=462166619&ptid=26689692][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]
x2 + 9x +  20.25 = (x + 4.5 )^2

我想知點解會係4.5 因為我計左係10.125

x2 - 1/2x + 0.0625 = (x + -0.25)^2

點解會係-0.025 我計到係 0.03125

x2 + 4px + 4p2 = (x+ 2p)^2

呢題我唔知4px 同 2p  ... [/quote]
You have to understand the format of the equation:
(x+y)^2 = x^2 + 2xy + y^2

In Q1, x^2 + 9x + 20.25
Then assume 2y is 9; then y is 4.5.
To confirm, (4.5)^2 is 20.25.

In Q2, assume 2y = 4p; then y = 2p.
To confirm, (2p)^2 is 4p^2.

This is good trick to save time in exam. But of course it's important to understand it.
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