SOLUTION: Hello! I need help with a few problems that I'm having difficulty with. 1) [(1+i)/i] - [(3)/(4-i)] For the common denominator, I got i(4-i)= 4i-1 Do you multiply this normally

Algebra ->  Trigonometry-basics -> SOLUTION: Hello! I need help with a few problems that I'm having difficulty with. 1) [(1+i)/i] - [(3)/(4-i)] For the common denominator, I got i(4-i)= 4i-1 Do you multiply this normally      Log On


   



Question 133075: Hello!
I need help with a few problems that I'm having difficulty with.
1) [(1+i)/i] - [(3)/(4-i)]
For the common denominator, I got i(4-i)= 4i-1
Do you multiply this normally or do you need a conjugate?
2) (sqrt-15) * (sqrt-15)
I really do not understand this problem.
Any help would be greatly appreciated! Thank you!

Found 2 solutions by solver91311, Edwin McCravy:
Answer by solver91311(24713) About Me  (Show Source):
You can put this solution on YOUR website!
%28%281%2Bi%29%2Fi%29+-+%28%283%29%2F%284-i%29%29

On the common denominator, you missed it by THAT much...

i%284-i%29=4i-%28i%5E2%29=4i-%28-1%29=4i%2B1=1%2B4i, so:



Now you need the conjugate of the denominator in order to rationalize the denominator:



For your other problem, I think you mean:

sqrt%28-15%29sqrt%28-15%29, which is %28i%2Asqrt%2815%29%29%28i%2Asqrt%2815%29%29=i%5E2%2A15=-15

Answer by Edwin McCravy(20054) About Me  (Show Source):
You can put this solution on YOUR website!
I need help with a few problems that I'm having difficulty with.
1) %281%2Bi%29%2Fi+-+3%2F%284-i%29
For the common denominator, I got i%284-i%29 = 4i-1

------------------------------
That's wrong. i%284-i%29 = 4i-i%5E2 = 
Replace i%5E2 by -1
4i-%28-1%29 = 4i%2B1 = 1%2B4i
------------------------------
But you shouldn't have tried to multiply it out yet anyway.
------------------------------

Do you multiply this normally or do you need a conjugate?

------------------------------

Here's the whole thing, starting from scratch:

%281%2Bi%29%2Fi+-+3%2F%284-i%29

The LCD = i(4-i)

DO NOT MULTIPLY THAT OUT YET!  

The denominator of the first fraction needs the factor %284-i%29
in order to become the LCD, so we multiply it by %284-i%29%2F%284-i%29
which is just 1, so it won't affect the value to multiply the first
fraction by that:

+%28+%281%2Bi%29%2Fi+%29%28+%284-i%29%2F%284-i%29+%29+-+3%2F%284-i%29++

The denominator of the second fraction needs the factor i
in order to become the LCD, so we multiply it by i%2Fi
which is just 1, so it won't affect the value to multiply the second
fraction by that:

+%28+%281%2Bi%29%2Fi+%29%28+%284-i%29%2F%284-i%29+%29+-+%283%2F%284-i%29%29%28i%2Fi%29

Indicate the multiplication of the numerators and the denominators:

+%28%281%2Bi%29%284-i%29%29%2F%28i%284-i%29%29+-+%283i%29%2F%28%284-i%29i%29

Now use FOIL on the left top:

%281%2Bi%29%284-i%29 = 4-i%2B4i-i%5E2 = 4%2B3i-i%5E2 = 
Now replace i%5E2 by -1
4%2B3i-%28-1%29 = 4%2B3i%2B1 = 5%2B3i and replace the left top by that:

+%285%2B3i%29%2F%28i%284-i%29%29+-+%283i%29%2F%28%284-i%29i%29
 
Distribute the left bottom

i%284-i%29 = 4i-i%5E2 = 
Replace i%5E2 by -1
4i-%28-1%29 = 4i%2B1 = 1%2B4i and
replace the left bottom by that:

+%285%2B3i%29%2F%281%2B4i%29+-+%283i%29%2F%28%284-i%29i%29

The right bottom is the same as the left bottom:

%284-i%29i = i%284-i%29 = 4i-i%5E2 =
Replace i%5E2 by -1
4i-%28-1%29 = 4i%2B1 = 1%2B4i 

So replace the right bottom by that:

+%285%2B3i%29%2F%281%2B4i%29+-+%283i%29%2F%281%2B4i%29

Now combine the numerators over the common denominator:

+%285%2B3i-3i%29%2F%281%2B4i%29

Collect like terms on top:

+5%2F%281%2B4i%29

Now we are ready to use the conjugate of the bottom.
We multiply by %281-4i%29%2F%281-4i%29 which again just
equals to 1 so it will not affect the value:

+%285%2F%281%2B4i%29%29%28%281-4i%29%2F%281-4i%29%29

Indicate the multiplication of the numerators and
denominators:

+%285%281-4i%29%29%2F%28%281%2B4i%29%281-4i%29%29

Distribute the top and use FOIL on the bottom:

+%285-20i%29%2F%281-4i%2B4i-16i%5E2%29%29

Combine like terms:

+%285-20i%29%2F%281-16i%5E2%29%29

Replace i%5E2 by -1

+%285-20i%29%2F%281-16%28-1%29%29%29

+%285-20i%29%2F%281%2B16%29

Combine like terms:

+%285-20i%29%2F%2817%29

Make two fractions:

5%2F17-%2820i%29%2F17

and write the answer in form A%2BBi

5%2F17-20%2F17i


2) %28sqrt%28-15%29%29+%2A+%28sqrt%28-15%29%29
I really do not understand this problem.
Any help would be greatly appreciated! Thank you!

Write -15 as %2815%29%28-1%29

+sqrt%28%2815%29%28-1%29%29+%2A+sqrt%28%2815%29%28-1%29%29 

Take individual square roots of factors under radicals

+sqrt%2815%29sqrt%28-1%29+%2A+sqrt%2815%29sqrt%28-1%29

Replace sqrt%28-1%29 by i

+sqrt%2815%29%2Ai+%2A+sqrt%2815%29%2Ai

%28sqrt%2815%29%29%5E2i%5E2

Replace %28sqrt%2815%29%29%5E2 by 15 and i%5E2 by -1

%2815%29%2A%28-1%29

-15

Edwin