F3二元1次方程數學

X - Y = 6 is correct.
(X)(X) = 2Y - 60 is wrong. It should be (X)(X) - 2Y = 60
that is (X)(X) = 2Y + 60.
So X^2 = 2(X - 6) + 60
X^2 = 2X - 12 + 60
X^2 - 2X - 48 = 0
(X + 6)(X - 8) = 0
X = - 6 (rej.)
X = 8.

X^2 + 7X = 15
X^2 + 7X - 15 = 0
You have to use formula or use calculator to get the answer.



2009-07-12 10:17:33 補充：
Q1. Y = X - 6 = 2.

2009-07-12 10:21:54 補充：
Q2. Question is OK, answer are : - 8.72 and 1.72 ( to 3 sig. fig.)

相差60歲可以是
XxX=2Y-60
亦可以是
2Y=XxX-60