# Question about geometric series

• Jun 1st 2010, 12:38 PM
yida
Question about geometric series
I don't quite understand half-life. How to solve this question?
http://www.mathhelpforum.com/math-he...56ygrthbrh.jpg
• Jun 1st 2010, 01:54 PM
skeeter
Quote:

Originally Posted by yida
I don't quite understand half-life. How to solve this question?
http://www.mathhelpforum.com/math-he...56ygrthbrh.jpg

$\displaystyle y = y_0\left(\frac{1}{2}\right)^{\frac{t}{h}}$

$\displaystyle y$ = amount left at any time $\displaystyle t$ in hrs

$\displaystyle y_0$ = amount at $\displaystyle t = 0$

$\displaystyle h$ = half-life in hrs
• Jun 1st 2010, 02:27 PM
yida
the answer is wrong
• Jun 1st 2010, 03:54 PM
skeeter
Quote:

Originally Posted by yida
the answer is wrong

what answer are you speaking of ?

I gave you the general function for half-life, not an "answer".
• Jun 1st 2010, 03:58 PM
yida
I use y0=55
t=12 h=7.2
but I got wrong answer
• Jun 1st 2010, 04:01 PM
skeeter
what "wrong answer" did you get?
• Jun 1st 2010, 04:04 PM
yida
55(0.5)^(12/7.2)
• Jun 1st 2010, 04:07 PM
skeeter
Quote:

Originally Posted by yida
55(0.5)^(12/7.2) = 17.324 mg

... and that is not wrong.