How to multiply complex numbers
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SAT Math › How to multiply complex numbers
has 4 roots, including the complex numbers. Take the product of
with each of these roots. Take the sum of these 4 results. Which of the following is equal to this sum?
The correct answer is not listed.
Explanation
This gives us roots of
The product of with each of these gives us:
The sum of these 4 is:
What we notice is that each of the roots has a negative. It thus makes sense that they will all cancel out. Rather than going through all the multiplication, we can instead look at the very beginning setup, which we can simplify using the distributive property:
Find the product of (3 + 4i)(4 - 3i) given that i is the square root of negative one.
7 + i
0
12 - 12i
24
24 + 7i
Explanation
Distribute (3 + 4i)(4 - 3i)
3(4) + 3(-3i) + 4i(4) + 4i(-3i)
12 - 9i + 16i -12i2
12 + 7i - 12(-1)
12 + 7i + 12
24 + 7i
Simplify:
Explanation
Use FOIL:
Combine like terms:
But since , we know
Evaluate .
None of the other choices gives the correct response.
Explanation
Apply the Power of a Product Rule:
,
and
,
so, substituting and evaluating:
Raise to the power of 3.
Explanation
To raise any expression to the third power, use the pattern
Setting :
Taking advantage of the Power of a Product Rule:
Since ,
and
:
Collecting real and imaginary terms:
Remember that .
Simplify:
Explanation
Use FOIL to multiply complex numbers as follows:
Since , it follows that
, so then:
Combining like terms gives:
Raise to the power of 3.
None of the other choices gives the correct response.
Explanation
To raise any expression to the third power, use the pattern
Setting :
Taking advantage of the Power of a Product Rule:
Since ,
and
:
Collecting real and imaginary terms:
;
is the complex conjugate of
.
Evaluate
.
Explanation
conforms to the perfect square trinomial pattern
.
The easiest way to solve this problem is to add and
, then square the sum.
The complex conjugate of a complex number is
.
,
so is the complex conjugate of this;
,
and
Substitute 8 for :
.
Evaluate
None of the other choices gives the correct response.
Explanation
Apply the Power of a Product Rule:
Applying the Product of Powers Rule:
raised to any multiple of 4 is equal to 1, and
, so, substituting and evaluating:
This is not among the given choices.
;
is the complex conjugate of
.
Evaluate
.
Explanation
conforms to the perfect square trinomial pattern
.
The easiest way to solve this problem is to subtract and
, then square the difference.
The complex conjugate of a complex number is
.
,
so is the complex conjugate of this;
Taking advantage of the Power of a Product Rule and the fact that :