數學知識交流---二元方程

為何
y³+17.52y-418.147=0
會變成
(10x-67)(0.1x²+0.67x+6.241)=0？
最後答案也錯。

2011-09-21 19:22:55 補充：
解：
x² - y² = 17.52 ---------(1)
2x²y = 836.294 ---------(2)
由(1)，得：
x² = 17.52 + y² ---------(3)
把(3)代入(2)，得：
2y (17.52 + y²) = 836.294
(y² + 17.52) y = 418.147
y³ + 17.52y - 418.147 = 0
(y - 6.7)(y² + 6.7y + 62.41) = 0
所以y = 6.7 或 y² + 6.7y + 62.41 = 0 (Δ<0，根為虛根，不能比較大小)
把y = 6.7代入(3)，得：
x² = 17.52 + 6.7²
x² = 62.41
x1 = 7.9(不合), x2 = -7.9
所以原方程組的解為x = 7.9, y = 6.7。

2011-09-21 19:25:10 補充：
所以原方程組的解為x = -7.9, y = 6.7。

7>x>-10------(1)
x²-y²=17.52------(2)
x²y+x²y=836.294------(3)
From (3),
2x²y=836.294
x²y=418.147------(4)
From (2).
x²=17.52+y²------(5)
Sub (5) into (4)
(17.52+y²)y=418.147
y³+17.52y-418.147=0
(10x-67)(0.1x²+0.67x+6.241)=0
x=67/10 or [-0.67±√(-2.0475)]/2 {rej. [From (1),(it involve complex number)]}
From (5),
y²=x²-17.52
y=±√(x²-17.52)------(6)
Sub x=67/10 into (6)
y=±√(4489/100-17.52)
y=±√27.37
Sub y=±27.37 into (5)
x²=17.52+27.37
x=±√44.89
x=±6.7

So, x=±6.7, y=±√27.37

'yx^2 + yx^2 = 836.294'??


is it wrong?

2011-09-07 19:28:01 補充：
two 'yx^2' ??!!

2011-09-07 19:48:21 補充：
so u mean 2yx^2=836.294, right?

2011-09-07 20:23:52 補充：
7>x>-10------(1)
x²-y²=17.52------(2)
x²y+x²y=836.294------(3)
From (3),
2x²y=836.294
x²y=418.147------(4)
From (2).
x²=17.52+y²------(5)
Sub (5) into (4)
(17.52+y²)y=418.147
y³+17.52y-418.147=0
(10x-67)(0.1x²+0.67x+6.241)=0
x=67/10 or [-0.67±√(-2.0475)]/2 {rej. [From (1),(it involve complex number)]}
From (5),
y²=x²-17.52
y=±√(x²-17.52)------(6)
Sub x=67/10 into (6)
y=±√(4489/100-17.52)
y=±√27.37
Sub y=±27.37 into (5)
x²=17.52+27.37
x=±√44.89
x=±6.7

So, x=±6.7, y=±√27.37

2011-09-15 21:12:17 補充：
To kon******:

U COPY MY ANSWER??!!!!!

No , it isn't wrong .

2011-09-07 19:51:24 補充：
You're right .