HP 15c Scientific Example: Branching and Looping

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 131I, a radioisotope. Write a program to figure the radioactivity at 3-day intervals until a given limit is reached. The formula for Nt, the amount of radioisotope remaining after t days, is

Nt = No (2-t/k),

where k = 8 days, the half-life of 131I, and N0 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 t1 – the first day of measurement – is stored in R0, N0

–the initial amount of isotope – is stored in R1, and the limit value for radioactivity is stored in R2.

Keystrokes	Display
¥	000-		Program mode.
´CLEAR M	000-		(Optional.)
´bA	001-42,21,11		Each loop returns to this
			line.
l0	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	2–t/k.
Y	009-	14