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# tricks for units use in HP Prime

Some tricks regarding using units in HP Prime.

Units are one of the main differences between scientific and graphical calculators. The aim of a good teacher is that students know what they are doing. When studying together with my sons, we always stress that they need to understand what the result is. “The answer is 50”, they would say. “50 what?”, I retort. This is the ultimate understanding of the problem being solved. Many times, a student fails to comprehend what he has obtained, applying the rules he has been given to solve the problems.

Graphical calculators have that: they allow you to dimension correctly your problem.

(in the discussion that follows, we will assume that we are in the home section of the calculator, and that RPN entry has been selected).

You enter the distance in meters, you enter the time in seconds, you divide both and you get the speed in m/s. (To get the units on the numbers in the Prime, you just write the number, go to the units menu, units submenu, the type of unit you want (length, for example), and the specific unit you need (km, m, cm, in, etc.). It’s that simple!)

Once you have your result in the right units, you can interpret what it is, and then change units if needed. To do so, you just need to supply the unit you want (just select the units menu, the units submenu, the type of unit you want and the specific unit you want to convert to. The prime will add the 1 in front, and you will get 1_km/h in the lower register of the stack.

To perform the conversion, you just need to go to the units menu, then tools, then convert. The Prime understand that you want to convert the quantity in the y register (level 2) to the units in the x register (level 1).

In some cases, you end up with a complex unit, having force, mass, time, whatever. What is it? The best you can do in this case is to have the Prime find the international system equivalent of whatever you have in X, using the MKSA function in the units/tools menu. Then you can understand what the hell is that!

One last thing: and now, how to remove the units from the number? you just obtained what you wanted, and now want to continue calculating with it, but you don’t want to continue keeping track of units. You can divide it by 1_the unit you just obtained - but it can take some time to write it, and you’d better rewrite the number instead!

The other (best?) alternative is to use the CAS.left() function on you previous result. It leaves the number without the unit!

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