数學不會做2

1). 7x^2-12x-4=0
(7x+2)(x-2) = 0
7x+2 = 0 or x-2 = 0
7x = -2 or x = 2
x = -2/7
i.e.. x = -2/7 or x = 2

2). x(3x+8) = 0
x=0 or 3x+8 = 0
3x = -8
x = -8/3
i.e.. x=0 or x = -8/3

3). let x is the smallest consective odd no.
x+(x+2)+(x+4)+(x+6) = 120
4x + 12 = 120
4x = 120-12
x = 108 / 4
x = 27
i.e... the smallest odd no is 27

對於第三題，可以有另一個設法，就係
設四個數係 x-3, x-1, x+1, x+3

咁樣一加埋之後，就得返 4x=120。
解完 x=30之後，最小奇數為 x-3 = 27。