Question

A polynomial ψ(s) = ansn + an-1sn-1 + ......+ a1s + a0 of degree n > 3 with constant real coefficients an, an-1, ... a0 has triple roots at s = -σ. Which one of the following conditions must be satisfied?

Options :

  1. ψ(s) = 0 at all the three values of s satisfying s3 + σ= 0

  2. ψ(s) = 0,  d ψ ( s ) d s = 0 and  d 2 ψ ( s ) d s 2 = 0 at s = -σ

  3. ψ(s) = 0,  d 2 ψ ( s ) d s 2 = 0 and  d 4 ψ ( s ) d s 4 = 0 at s = -σ

  4. ψ(s) = 0,  d 3 ψ ( s ) d s 3 = 0 at s = -σ

Show Answer

Answer :

ψ(s) = 0,  d ψ ( s ) d s = 0 and  d 2 ψ ( s ) d s 2 = 0 at s = -σ

Solution :

Polynomial, ψ(s) = ansn + an-1sn-1 + ......+ a1s + a0 of degree n > 3

It has triple root at s = -σ

so, at s = -σ

ψ(s) will be equal to zero, because it is a root of the polynomial

ψ(s) = ψ(-σ) = 0

Since polynomial ψ(s) has triple root at s = -σ

So, (s + σ)3 is one factor of polynomial

Since the factor has a cube, it will have an inflexion point in the curve as the x3 has.

Hence first and second-order derivatives will be zero at the point of inflexion.

d ψ ( s ) d s = 0 , d 2 ψ ( s ) d s 2 = 0

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