SOLUTION: Can you please help me solve this? I'm supposed to solve for all roots but I'm not sure how. {{{8x^3+27=0}}}

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Question 189584: Can you please help me solve this? I'm supposed to solve for all roots but I'm not sure how. 8x%5E3%2B27=0
Found 2 solutions by jim_thompson5910, Edwin McCravy:
Answer by jim_thompson5910(35256) About Me  (Show Source):
You can put this solution on YOUR website!
First, we need to factor 8x%5E3%2B27


8x%5E3%2B27 Start with the left side of the equation.


%282x%29%5E3%2B%283%29%5E3 Rewrite 8x%5E3 as %282x%29%5E3. Rewrite 27 as %283%29%5E3.


%282x%2B3%29%28%282x%29%5E2-%282x%29%283%29%2B%283%29%5E2%29 Now factor by using the sum of cubes formula. Remember the sum of cubes formula is A%5E3%2BB%5E3=%28A%2BB%29%28A%5E2-AB%2BB%5E2%29


%282x%2B3%29%284x%5E2-6x%2B9%29 Multiply


So 8x%5E3%2B27 factors to %282x%2B3%29%284x%5E2-6x%2B9%29.


In other words, 8x%5E3%2B27=%282x%2B3%29%284x%5E2-6x%2B9%29


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So the equation 8x%5E3%2B27=0 then becomes %282x%2B3%29%284x%5E2-6x%2B9%29=0


%282x%2B3%29%284x%5E2-6x%2B9%29=0 Start with the factored equation.


2x%2B3=0 or 4x%5E2-6x%2B9=0 Set each factor equal to zero


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Let's solve the first equation 2x%2B3=0


2x%2B3=0 Start with the first equation.


2x=-3 Subtract 3 from both sides.


x=-3%2F2 Divide both sides by 2.


So the first answer is x=-3%2F2

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Now let's solve the second equation 4x%5E2-6x%2B9=0


Notice we have a quadratic equation in the form of ax%5E2%2Bbx%2Bc where a=4, b=-6, and c=9


Let's use the quadratic formula to solve for x


x+=+%28-b+%2B-+sqrt%28+b%5E2-4ac+%29%29%2F%282a%29 Start with the quadratic formula


x+=+%28-%28-6%29+%2B-+sqrt%28+%28-6%29%5E2-4%284%29%289%29+%29%29%2F%282%284%29%29 Plug in a=4, b=-6, and c=9


x+=+%286+%2B-+sqrt%28+%28-6%29%5E2-4%284%29%289%29+%29%29%2F%282%284%29%29 Negate -6 to get 6.


x+=+%286+%2B-+sqrt%28+36-4%284%29%289%29+%29%29%2F%282%284%29%29 Square -6 to get 36.


x+=+%286+%2B-+sqrt%28+36-144+%29%29%2F%282%284%29%29 Multiply 4%284%29%289%29 to get 144


x+=+%286+%2B-+sqrt%28+-108+%29%29%2F%282%284%29%29 Subtract 144 from 36 to get -108


x+=+%286+%2B-+sqrt%28+-108+%29%29%2F%288%29 Multiply 2 and 4 to get 8.


x+=+%286+%2B-+6i%2Asqrt%283%29%29%2F%288%29 Simplify the square root (note: If you need help with simplifying square roots, check out this solver)


x+=+%286%2B6i%2Asqrt%283%29%29%2F%288%29 or x+=+%286-6i%2Asqrt%283%29%29%2F%288%29 Break up the expression.


x+=+%283%2B3i%2Asqrt%283%29%29%2F%284%29 or x+=+%283-3i%2Asqrt%283%29%29%2F%284%29 Reduce


So the other two answers are x+=+%283%2B3i%2Asqrt%283%29%29%2F%284%29 or x+=+%283-3i%2Asqrt%283%29%29%2F%284%29


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Answer:


So the three solutions are x=-3%2F2, x+=+%283%2B3i%2Asqrt%283%29%29%2F%284%29, or x+=+%283-3i%2Asqrt%283%29%29%2F%284%29

Answer by Edwin McCravy(20059) About Me  (Show Source):
You can put this solution on YOUR website!
Can you please help me solve this? I'm supposed to solve for all roots but I'm not sure how. 8x%5E3%2B27=0

You must learn the principle of factoring the sum and difference of
two cubes:

FIRST%5E3+%2B+SECOND%5E3 

factors as

%28FIRST%2BSECOND%29%28FIRST%5E2-FIRST%2ASECOND%2BSECOND%5E2%29

and

FIRST%5E3+-+SECOND%5E3 

factors as

%28FIRST-SECOND%29%28FIRST%5E2%2BFIRST%2ASECOND%2BSECOND%5E2%29

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Your problem is the first case as it has a + sign between
the two terms:

8x%5E3%2B27=0

Notice that 8x%5E3 can be written as %282x%29%5E3

Notice that 27 can be written as %283%29%5E3

So we write it as 

%282x%29%5E3%2B%283%29%5E3=0

So FIRST=2x and SECOND=3

and since

FIRST%5E3+%2B+SECOND%5E3 

factors as

%28FIRST%2BSECOND%29%28FIRST%5E2-FIRST%2ASECOND%2BSECOND%5E2%29

we factor the left side of 

%282x%29%5E3%2B%283%29%5E3=0

as

%282x%2B3%29%28%282x%29%5E2-%282x%29%2A%283%29%2B%283%29%5E2%29=0%29

Simplifying inside the second parentheses:

%282x%2B3%29%28%282%5E2%29%28x%5E2%29-%286x%29%2B9%29=0

%282x%2B3%29%284x%5E2-6x%2B9%29=0

So we use the zero factor property:

2x%2B3=0,  4x%5E2-6x%2B9=0

Solving the first,

2x%2B3=0
2x=-3
x=-3%2F2

So one root is -3%2F2 

Solving the second:

4x%5E2-6x%2B9=0

That doesn't factor so we resort to the
quadratic formula:

x+=+%28-b+%2B-+sqrt%28+b%5E2-4%2Aa%2Ac+%29%29%2F%282%2Aa%29+ 

where a=4, b=-6, c=9

x+=+%28-%28-6%29+%2B-+sqrt%28+%28-6%29%5E2-4%2A%284%29%2A%289%29+%29%29%2F%282%2A%284%29%29+ 

x+=+%286+%2B-+sqrt%2836-144%29+%29%2F8+ 

x+=+%286+%2B-+sqrt%28-108%29+%29%2F8+

x+=+%286+%2B-+i%2Asqrt%28108%29+%29%2F8+

x+=+%286+%2B-+i%2Asqrt%2836%2A3%29+%29%2F8+

x+=+%286+%2B-+6i%2Asqrt%283%29+%29%2F8+

Make two fractions on the right

x+=+6%2F8+%2B-+%286i%2Asqrt%283%29%29%2F8

Reduce the fractions by dividing top and 
bottom by 2:

x+=+3%2F4+%2B-+%283i%2Asqrt%283%29%29%2F4

So the other two roots are

3%2F4+%2B+%283sqrt%283%29%29%2F4i

and 

3%2F4+-+%283sqrt%283%29%29%2F4i

Edwin