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Find the sum of the reciprocals of the roots of x^2-13x+4=0.

 Dec 23, 2018
 #1
avatar+4625 
+4

Hint:  Use Vieta's

 Dec 23, 2018
 #2
avatar+776 
+3

Using the quadratic formula, we have the two solutions 132+3217 and 1323217. The reciprocals would be 1132+3217  and 11323217. Adding them together, we have (1(132+3217))+(1(1323217))=134.

 

- PM

 

wink

 Dec 23, 2018
edited by PartialMathematician  Dec 23, 2018
 #3
avatar+776 
+3

But yes, a good solution would include the use of Vieta's formula. 

 

In the quadratic polynomial  P(x)=ax2+bx+c, the roots x1 and x2 satisfy x1+x2=ba and x1x2=ca.

PartialMathematician  Dec 23, 2018
 #4
avatar+776 
+3

Note that in this equation, x1+x2=13 and x1x2=4.

PartialMathematician  Dec 23, 2018
 #6
avatar+776 
+3

So the answer is (1(132+3217))+(1(1323217))=134.

 

- PM

PartialMathematician  Dec 23, 2018
 #5
avatar+130475 
+5

The roots are

 

13 + 3√17                        13 - 3√17

_________     and          _________

     2                                       2

 

So

 

The reciprocal  sum is

 

      2                       2

_________  +    _________

13 + 3√17            13 - 3√17

 

 

  2 [ 13 - 3√17]  + 2 [ 13 + 2√13 ]

__________________________

        169  - 9(17)

 

 

  52              13

____  =       ___

  16               4

 

 

 

 

cool cool cool

 Dec 23, 2018
 #7
avatar+818 
+3

Thank you, everyone!

 Dec 24, 2018

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