HP -28S Manual page 24

Add 15 and 23.
[B
Ii
The numbers 15 and 23 are removed from the stack, and their sum,
38, is returned to level 1. You'll leave this intermediate result on the
stack while you calculate the second intermediate result.
To calculate sin 30° you'll use the TRIG (trigonometry) menu.
ITRIGI
3:
2:
1: 38
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The bottom line of the display shows six commands in the TRIG
menu. The six menu labels (
$Itt
through
ATA" )
define the six
menu keys (the keys immediately below the display).
Press
W []]
to write 30 in the command line.
30
Put 30 in level 1.
I
ENTER
I
3:
2: 38
1: 30
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The previous result, 15
+
23
=
38, is lifted to level 2.
Calculate sin 30°.
SUI
3:
2:
38
1: .5
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1: Getting Started 23