Help me about calculate.

[sidercho]

Hello guys I ve been workin on since a few days.How can I calculate it?


I5 = ((15.497 + j0.888)*(30-j25))/ ((30+j25)*(30-j25))

 

[Deleted member 4993]

Hello guys I ve been workin on since a few days.How can I calculate it?


I5 = ((15.497 + j0.888)*(30-j25))/ ((30+j25)*(30-j25))

Please show us what you have tried and exactly where you are stuck.

Please follow the rules of posting in this forum, as enunciated at:

https://www.freemathhelp.com/forum/threads/read-before-posting.109846/#post-486520

Please share your work/thoughts about this assignment.

hint: (assuming $\displaystyle j = \ \sqrt{-1}$ )

j * j = -1

 

[HallsofIvy]

It looks to me like you started with $\displaystyle \frac{15.497+ j0.888}{30+ j25}$
and the multiplied both numerator and denominator by $\displaystyle 30- j25$.

Of course multiplying both numerator and denominator of any fraction by the same thing doesn't change the value of the fraction! But it is standard to do that to fractions of complex numbers. We are multiplying numerator and denominator by the complex conjugate of the denominator because $\displaystyle (c+ jd)(c- jd)= c^2- jcd+ jcd- j^2d^2= c^2+ d^2$, a positive real number. Note that -jcd+ jcd cancel while $\displaystyle j^2d^2= -d^2$.

In this particular problem, the denominator will be $\displaystyle (30+ j25)(30- j25)= 30^2+ 25^2$, a real number. So the problem is now to multiply $\displaystyle (15.497+ j0.888)(30- j25)= (15.497)(30)- j(15.497)(25)+ j(0.888)(30)+ (0.888)(25)$ and then divide by the real number denomimator.

 

[Steven G]

I am not sure what the I5 on the left means.

If it is meant to be 15, then I doubt that this has to do with complex numbers and that j is just a variable.

If I5 means that I*5, then I guess that j is sqrt(-1) and to find I we must divide the rhs by 5

The OP did say to calculate it. That confuses things even more.

 

[Deleted member 4993]

The original post went up on Wednesday - today is Saturday - no response from OP.