HP 15C problem


Help! My calculator will only give answers to trig problems in radians. No matter what the mode is. Also, there is a small "C" in the lower right quadrant. ~?

Thanx, I hope.




your HP15C is set to work with comples numbers (x+iy), and in this mode it ´understands´ angles in radians, only.

Please, locate the [CF] inscription in blue, in the slanted face of the [5] key. Now press:

[g] [CF] 8
This should bring your HP15C to real-mode operation.


Luiz (Brazil)

Edited: 9 Apr 2007, 3:22 p.m.


Also, there is a small "C" in the lower right quadrant.

The calculator is in complex mode. Press [g] [CF] [8] to leave this mode, in case you haven't done this yet.



two simultaneous answers---and both from Brasil no less!


Tom --

Gerson and Luiz have already answered the question, and provided the remedy. I don't know if you have a manual, but a PDF scan can be purchased on a CD or DVD for a nominal price from this website. A print copy on eBay will cost US$25-35.

Flag 8 set puts the calculator in "Complex mode", in which the input arguments and output results are complex-valued where applicable. It's one of the few things that are not very intuitive on the HP-15C. Flag 8 is testable, allowing user programs to account for results in real and complex mode. Other mode settings -- such as angular measure (DEG/RAD/GRD) or display (FIX/SCI/ENG) -- are not testable on the HP-15C.

So, why is radians mode enforced when Complex mode is set?

Answer: Trigonometric functions are mathematically defined for complex-valued arguments. Their formulae that involve hyperbolic functions, whose arguments must be dimensionless and unscaled (as radians are). For example:

sin (a + ib) = (sin a)*(cos ib) +   (cos a)*(sin ib)
= (sin a)*(cosh b) + i*(cos a)*(sinh b)

cos (a + ib) = (cos a)*(cos ib) - (sin a)*(sin ib)
= (cos a)*(cosh b) - i*(sin a)*(sinh b)

Obviously, with b = 0, sine or cosine in complex mode (or these expressions) will produce results identical to sine or cosine in real mode with input in radians, but will take longer to execute.

-- KS

Edited: 9 Apr 2007, 11:00 p.m. after one or more responses were posted


Hi, I spotted scanned pdf manuals for 15c on

The 'Owner's Handbook' is hidden under button before 'Documentation' line and the 'Advanced Functions Handbook' under dot at the end of this line.



Hi, I spotted scanned pdf manuals for 15c on ...

Yes, I was aware of the availability of the manuals at that site. I didn't want to advertise it. That site host probably bought one copy of CD #1 from MoHPC, then made the manuals freely available to the world without permission to redistrubute. There is certainly a reasons that the links are so (ahem) "subtle". It probably steals sales of MoHPC CD's and DVD's.

The issue was discussed a few months ago:


-- KS


Hi, I didn't know about that discussion and I found the page and pdf files accidentally when I was looking for other stuff. When I found that, I didn't notice any signs inside those files that it could belong to MoHPC CD's and DVD's. When you look inside those files you can hardly expect that it is based on work that somebody want to protect (even if such information was removed from the original files). Moreover, Tom apparently owns the calculator, therefore I didn't expect any reason why he shouldn't download those files. So, you can expect that somebody time to time will post this link with the best intention to help also in the future. Just because there is no way how to spot it could be problematic. If somebody takes it as problem he definitely should solve it with the owner of the site and not with anybody else.

Best Regards,



I've tried to avoid the issue because I thought Chris' heart was in the right place with wanting to bring back the HP-15C and all, but it is growing into an issue. It also appears to be becoming an issue at the HP level. The current calculator team recently asked me to show them the permission I received from an earlier team. When I was at HP recently, someone told me that they would like to clean up a lot of infringement on the net but they don't want to take down a lot of "good" sites with the "bad" sites.

For future reference, for the last 30 years or so, it hasn't been necessary to mark anything as copyrighted. This is the default. Instead you should look for a statement of the work being in the public domain etc. (This isn't meant to imply that you did something bad - just an FYI.)

Edited: 9 Apr 2007, 3:59 p.m.


When I bought a used 15c some years ago, it came with a CD with the whole manual scanned. Each page was a separate PDF file with rather low resolution and the pages were often skewed. I put part of it together and when I lent my 15c to Ben, I sent a copy of that manual along. I don't think it was the same scan as MoHPC. So perhaps there are a number of scans out there?


Yeah I bought some software from a major company on ebay recently and it came on a CD-R with an ink-jet printed label. Even after I questioned that, the seller claimed it was a legit "Corporate Disk". The software maker said it wasn't. The seller was happy to let me return it, so I did and it bounced back because his street address didn't exist. That account was closed down but I'll bet he has another one by now.

There's lots of stuff going on.

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