Questions on Algebra: Quadratic Equation answered by real tutors!

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Question 167476: I need to solve by completing the square.I'm not sure how to do 1 half of negative s. The problem is s^2-s-6=0: I need to solve by completing the square.I'm not sure how to do 1 half of negative s. The problem is s^2-s-6=0
Answer by jim_thompson5910(9862) About Me  (Show Source):
You can put this solution on YOUR website!

s^2-s-6 Start with the left side of the equation.


Take half of the s coefficient -1 to get -1/2. In other words, (1/2)(-1)=-1/2.


Now square -1/2 to get 1/4. In other words, (-1/2)^2=(-1/2)(-1/2)=1/4


s^2-s+highlight(1/4-1/4)-6 Now add and subtract 1/4. Make sure to place this after the "s" term. Notice how 1/4-1/4=0. So the expression is not changed.


(s^2-s+1/4)-1/4-6 Group the first three terms.


(s-1/2)^2-1/4-6 Factor s^2-s+1/4 to get (s-1/2)^2.


(s-1/2)^2-25/4 Combine like terms.


So after completing the square, s^2-s-6 transforms to (s-1/2)^2-25/4. So s^2-s-6=(s-1/2)^2-25/4.


So s^2-s-6=0 is equivalent to (s-1/2)^2-25/4=0.

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(s-1/2)^2-25/4=0 Start with the completed square set to 0.


(s-1/2)^2=0+25/4 Add 25/4 to both sides.


(s-1/2)^2=25/4 Combine like terms.


x-1/2=0+-sqrt(25/4) Take the square root of both sides.


s-1/2=sqrt(25/4) or s-1/2=-sqrt(25/4) Break up the "plus/minus" to form two equations.


s-1/2=5/2 or s-1/2=-5/2 Take the square root of 25/4 to get 5/2. Note: (5/2)^2=25/4 so sqrt(25/4)=5/2


s=1/2+5/2 or s=1/2-5/2 Add 1/2 to both sides.


s=(1+5)/2 or s=(1-5)/2 Combine the fractions


s=6/2 or s=-4/2 Combine the numerators


s=3 or s=-2 Reduce.


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Answer:


So the solutions are s=3 or s=-2.