F.4 maths題目

Let two consecutive negative even numbers be x and x+2 :

then x(x+2) = 80

x^2 + 2x - 80 = 0

(x - 8)(x + 10) = 0

x = 8 (rejected since 8 is not negative number)) or

x = - 10

the larger number = - 10 + 2 = - 8

So the two consecutive negative even number are - 8 and - 10

呢個問題, 你要睇返條題目, The product of two consecutive negative number is 80.好明顯答案一定要係兩個負數, 姐係 -8 同 -10, 而其中就以-8較大, 所以答案係-8
你可能會有疑問
x(x+2) = 80
x^2 + 2x - 80 = 0
(x-8)(x+10) = 0
x = 8 or x = -10
你可能會問, -8何來?
請睇一睇以下例子:
-(x-8)(x+10) = 0
(x-8)*-(x+10) = 0
以上兩例都同原本一樣的, 因為-1可以除過去0到消去, 由此就可以得到,
其實答案有四個:8,10,-8,-10
而其他兩個正數會被rejected