SOLUTION: Find the sum of the cubes of the roots of {{{ x^2-2x+13=0 }}}, but do not find the roots.

Algebra ->  Quadratic Equations and Parabolas -> SOLUTION: Find the sum of the cubes of the roots of {{{ x^2-2x+13=0 }}}, but do not find the roots.       Log On


   



Question 927772: Find the sum of the cubes of the roots of +x%5E2-2x%2B13=0+, but do not find the roots.
Found 2 solutions by MathLover1, Edwin McCravy:
Answer by MathLover1(20849) About Me  (Show Source):
You can put this solution on YOUR website!

+x%5E2-2x%2B13=0+...use quadratic formula to find roots

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

x+=+%28-%28-2%29%2B-+sqrt%28+%28-2%29%5E2-4%2A1%2A13+%29%29%2F%282%2A1%29+


x+=+%282%2B-+sqrt%284-52%29%29%2F2+

x+=+%282%2B-+sqrt%28-48%29%29%2F2+
x+=+%282%2B-+sqrt%28%28-1%2948%29%29%2F2+
x+=+%282%2B-+6.93%2Ai%29%2F2+
complex solutions (means there is no x-intercepts):

x+=+%282%2B+6.93%2Ai%29%2F2+
x+=+2%2F2%2B+%286.93%2Ai%29%2F2+
x+=+1%2B+3.465%2Ai+
or
x+=+%282-6.93%2Ai%29%2F2+
x+=+2%2F2-%286.93%2Ai%29%2F2+
x+=+1-3.465%2Ai+

the sum of the cubes of the roots will be:
%281%2B+3.465%2Ai%29%5E3%2B%281-+3.465%2Ai%29%5E3

-35.0187-31.2066i-35.0187%2B31.2066i
-35.0187-cross%2831.2066i%29-35.0187%2Bcross%2831.2066i%29
-70.0374


Answer by Edwin McCravy(20054) About Me  (Show Source):
You can put this solution on YOUR website!
The other tutor did EXACTLY what you were told NOT to do!!!!
She found the roots!!!  She got an approximate version of the
answer, but the answer oames out exactly as an integer.

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

Here's how to do it as you were told to do it.

A "monic" polynomial is a polynomial whose leading coefficient is 1.

The coefficient of the next to largest power of x in a monic polynomial is
-1 times the sum of the roots.

The last term of a monic polynomial in x is the product of the roots if the
degree is even [and -1 times the product of the roots if the degree is odd.]

+x%5E2-2x%2B13=0+ is a monic polynomial of even degree 2.

Therefore the sum of its roots is 2 and the product of its roots is 13.

Let its roots be r%5B1%5D and r%5B2%5D

Then r%5B1%5D%2Br%5B2%5D%5D%22%22=%22%222 and r%5B1%5Dr%5B2%5D=13

%28r%5B1%5D%2Br%5B2%5D%29%5E3%22%22=%22%22
r%5B1%5D%5E3%2B3r%5B1%5D%5E2r%5B2%5D%2B3r%5B1%5Dr%5B2%5D%2Br%5B2%5D%5E3%22%22=%22%22
r%5B1%5D%5E3%2B3r%5B1%5Dr%5B2%5D%28r%5B1%5D%2Br%5B2%5D%29%2Br%5B2%5D%5E3%22%22=%22%22
%28r%5B1%5D%5E3%2Br%5B2%5D%5E3%29%2B3%2Ar%5B1%5Dr%5B2%5D%28r%5B1%5D%2Br%5B2%5D%29

So
%28r%5B1%5D%2Br%5B2%5D%29%5E3%22%22=%22%22%28r%5B1%5D%5E3%2Br%5B2%5D%5E3%29%2B3%28r%5B1%5Dr%5B2%5D%29%28r%5B1%5D%2Br%5B2%5D%29

Substitute: r%5B1%5D%2Br%5B2%5D%5D%22%22=%22%222 and r%5B1%5Dr%5B2%5D=13

2%5E3%22%22=%22%22%28r%5B1%5D%5E3%2Br%5B2%5D%5E3%29%2B3%2813%29%282%29

8%22%22=%22%22%28r%5B1%5D%5E3%2Br%5B2%5D%5E3%29%2B78

-70%22%22=%22%22r%5B1%5D%5E3%2Br%5B2%5D%5E3

Edwin