## 两个数字之差为14，总和为20。它们的乘积是多少？

• 两个未知数需要两个方程式三个未知数需要三个方程式，依此类推。

x-y = 14

x-y + y = 14 + y

x = 14 + y

（14 + y）+ y = 20

14 + 2y = 20

14-14 + 2y = 20-14

2y = 20-14

2y = 6

y = 3

x-y = 14

x-3 = 14

x-3 + 3 = 14 + 3

x = 14 + 3

x = 17

x * y = 17 * 3 = 51

x-y = 14

x + y = 20

x = y + 14

（y + 14）+ y = 20

2y + 14 = 20

2y = 6

y = 3

x + y = 20

x + 3 = 20

x = 27

x÷y = z

27 ÷ 3 = 9

$\text {Let the two numbers be x and y then}$

$x + y = 20 \qquad equation\:1$

$x - y = 14 \qquad equation\:2$

$\text{ by adding 1 and 2}\qquad 2 x = 34 \implies x = 17$

$\text{ by subtracting 2 from 1}\qquad 2 y = 6 \implies y = 3$

$\text{ Therefore the product of the numbers is 3(17) = 51}$