# How to do complex no.



## chako (Jun 23, 2010)

How to convert -120&lt;-210 , using fx115es ... for some reason calculator is not converting this value.

any help ....


----------



## pelaw (Jun 23, 2010)

I dont know about calculators, but why not just draw a circle and do it by hand?

-210 deg = 150 deg, so + 120 at 150 = + 120 at -210

But because you have (-) 120, the vector will be in the opposite direction (pointing towards the center of the circle)

So - 120 at 150 deg = 120 at -30 deg. So, -120&gt;-210 = 120&gt;-30, and this is simple to put in complex form.

In essence - 120 means that the vector is pointing towards the circle at the angle of - 210, which is the same as saying that the vector +120 is pointing away from the circle at an agle of - 30.


----------



## chako (Jun 24, 2010)

pelaw said:


> I dont know about calculators, but why not just draw a circle and do it by hand?
> -210 deg = 150 deg, so + 120 at 150 = + 120 at -210
> 
> But because you have (-) 120, the vector will be in the opposite direction (pointing towards the center of the circle)
> ...


Thanks for reply and suggetion... but when it comes to do multiple of them during exam.. calculator comes handy.... i was able to get it by hand.. but u have few calculations involving this -120 part.. it takes a while to do it... so any suggetion on how to do it in calculator quickly.... thanks..


----------



## ELEPE (Jun 24, 2010)

chako said:


> pelaw said:
> 
> 
> > I dont know about calculators, but why not just draw a circle and do it by hand?
> ...


Part of the FE exam is ability to use common sense instead of having your head buried in your calculator all the time. If you have more intuition about the problem sometimes it saves you valuable time and also stupid calculator mistakes. In this case -120 &lt; -210 is the same as 120 &lt; -30 since the minus sign is just a 180 degree phase shift. So you see if you understand the problem there is zero calculation. I hope you are not one of those that spend a week trying to punch in all the formulae into your calculator instead of spending time trying to understand.


----------



## pelaw (Jun 24, 2010)

Not only that, but how about developing routine to think in terms of cos, sin and tan-1.

-120 cos (-210) + j (-120) sin (-210) = 104 - j 60; and backwards

sqrt(104^2 + 60 ^2) &lt; tan-1(-60/104) = 120 &gt; -30


----------



## chako (Jun 28, 2010)

ELEPE said:


> chako said:
> 
> 
> > pelaw said:
> ...



Ofcourse I can do it in head and thats what i am doing... its not a big deal to solve this kind of values in head... but my question was just because i was curious to find out, why calculator is not accepting -values.... thats it... but thanks for the imput...


----------



## BamaBino (Jun 29, 2010)

chako said:


> How to convert -120&lt;-210 , using fx115es ... for some reason calculator is not converting this value. any help ....


I tried it on the fx115 and think I see the problem the OP was having.

The mag of a complex number can't be negative.

On fx115, enter it as: -(120&lt;-210)

= 104 -60j


----------

