/sci/ - Science & Math

- Sorry for maybe dumb question and weird English-

8/10 = 4/5, right?

But in probability theory, that don't make sence for me.

I mean: If you have 10 cards, numbered 1-10, it's 8/10 chance that i DON'T get 1 or 2.

But if you have 5 cards, numbered 1-5, its 3/5 chance that you DON'T get 1 or 2.

But 10/8 = 5/4 wasn't it?

But apology for this - perhaps - dumbest question..

Please help me with mu noobish quadratic functions..

x^2 + 5x - 14 = 0
x = -5/2 +- root of (5/2)^2 + 14
x = -5/2 +- root of 25/4 + 56/4
x = -5/2 +- root of 81/4
x = -5/2 +- 9/2

x.1 = -5/2 + 9/2 = 4/2 = 2
x.2 = -5/2 - 9/2 = -14/2 = -7

x.1 = 2
x.2 = -7

Where 56 come from?? Sorry for not using math language, im on my phone.. Thanks.