Math III Section 4.6 #29 and #31



Your friend is using Descartes's Rule of Signs to find the number of negative real roots of x^3 + x^2 + x + 1 = 0.

According to Decarte's rule of sign, the number of negative real roots of p(x)=0 is either equal to the number of sign changers between consecutive coefficients of p(-x) or is less than that by an even number.

"P(-x) = (-x)^3 +(-x)^2 + (-x) +1

= -x^3 - x^2 - x +1

  • ** This is wrong because when she changed from P(x) to P(-x) she switched the x^2 to -x^2. It should have been positive and because it would have been positive you would have had 3 sign changes, not 1.**

To find possibilities for negative real roots, count the number of sign changes in the equation for p(-x).



-x^3+x^2 - x+1      <----- Now count the sign changes

  • because there is 3 which is and odd number, you use 3 and count by odds to zero...
  • `so there is 3 or 1 negative real roots


A gardener is designing a new garden in the shape of a trapezoid. She wants the shorter base to be twice the height and the longer base to be 4 feet longer than the shorter base. If she has enough topsoil to create a 60ft^2 garden, what dimensions should she use for the garden?

h=5 , B1=14, B2=10

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