聯立2元1次問題

equation 1: 4x + 5y = 1
equation 2: 2x - y + 3 = 0

if equation 1 - 2 * equation 2, then

(4x + 5y) - 2 * (2x - y + 3) = (1) - 2 * (0)
(4x - 2 * 2x) + [5y - 2 * (-y)] - 2 * 3 = 1
7y - 6 = 1
7y = 7
y = 1

then put y = 1 into either equation 1 or equation 2, you can get
x = -1

4x+5y=1......(1)
2x-y+3=0......(2)

(2) * 2: 4x - 2y + 6 = 0 ......(3)

(1) - (3): 7y - 6 = 1
7y = 7
y=1

Sub y=1 into (2): 2x - 1+ 3 = 0
x = -1