HP 15c - Manual (Page 93)

Section 8: Program Branching and Controls 93
Examples
Example: Branching and Looping
A radiobiology lab wants to predict the
diminishing radioactivity of a test amount of
131
I, a radioisotope. Write a program to figure
the radioactivity at 3-day intervals until a
given limit is reached. The formula for N
t
, the
amount of radioisotope remaining after t days,
is
N
t
= N
o
(2
-t/k
),
where k = 8 days, the half-life of
131
I, and N
0
is the initial amount.
The following program uses a loop to calculate the number of millicuries
(mci) of isotope theoretically remaining at 3-day intervals of decay.
Included is a conditional test to check the result and end the program when
radioactivity has fallen to a given value (a limit).
The program assumes t
1
– the first day of measurement – is stored in R
0
, N
0
– the initial amount of isotope – is stored in R
1,
and the limit value for
radioactivity is stored in R
2
.
Keystrokes Display
| ¥ 000- Program mode.
´ CLEAR M 000- (Optional.)
´ b A 001-42,21,11 Each loop returns to this
line.
l 0 002- 45 0 Recalls current t which
changes with each loop.
´ © 003- 42 31 Pauses to display t.
8
004- 8 k
÷ 005- 10
“ 006- 16 –t/k.
2
007- 2
® 008- 34
Y 009- 14 2
–t/k
.