Subjective Type

If the equation $$x^{5} - 10a^{3}x^{2} + b^{4}x + c^{5} = 0$$ has three equal roots, show that $$ab^{4} - 9a^{5} + c^{5} = 0$$.

Solution

Let

$$f(x)=x^5-10a^3x^2+b^4x+c^5=(x-m)^3g(x)$$

f(x)=x5−10a3x2+b4x+c5=(x−m)3g(x)
where $$m$$ is the repeated root,
and $$g(x)$$is some second-order polynomial.
g(x
Then, differentiating and substituting $$x=m$$,

$$5x^4-20a^3x+b^4=(x-m)^3g'(x)+3(x-m)^2g(x)$$

$$5m^4-20a^3m+b^4=0\dots eqn(1)$$

5m4−20a3m+b4=0
Differentiating another time, and similarly substituting,$$x=m$$

$$20m^3-20a^3=0$$

$$m=a$$

Substituting this back into our equation
$$a^5-10a^5+b^4a+c^5=0\\ \Rightarrow ab^4-9a^5+c^5=0$$


SIMILAR QUESTIONS

Theory of Equations

From the top of a $$64$$ metres high tower, a stone is thrown upwards vertically with a velocity of $$48 \text{ m/s}$$. The greatest height (in metres) attained by the stone, assuming the value of gravitational acceleration $$g = 32 \text{ m/s}^2$$, is (Note: This question was asked in Maths subject in JEE Mains $$2015$$ online exam held on $$11$$ April $$2015$$)

Theory of Equations

The sum of all the real values of $$x$$ satisfying the equation $$2^{(x-1)(x^2+5x-50)}=1$$ is.

Theory of Equations

Let $$\alpha$$ and $$\beta$$ be the roots of equation $$x^2-6x-2=0$$. If $$a_n=\alpha^n-\beta^n$$, for $$n\geqslant 1$$, then the value of $$\dfrac {a_{10}-2a_8}{2a_9}$$ is equal to

Theory of Equations

If $$2x^3+ax^2+bx+4=0$$ (a and b are positive real numbers) has $$3$$ real roots, then prove that $$ a+b \geq 6(2^{1/3}+4^{1/3})$$.

Theory of Equations

Show that the equation $$x^{4} - 12x^{2} + 12x - 3 = 0$$ has a root between $$-3$$ and $$-4$$ and another between $$2$$ and $$3$$.

Theory of Equations

Show that $$x^{5} + 5x^{4} - 20x^{2} - 19x - 2 = 0$$ has a root between $$2$$ and $$3$$, and a root between $$-4$$ and $$-5$$.

Theory of Equations

Find the solutions of the following equations which have common roots: $$4x^{4} + 12x^{3} - x^{2} - 15x = 0, 6x^{4} + 13x^{3} - 4x^{2} - 15x = 0$$.

Theory of Equations

Find the condition that $$x^{n} - px^{2} + r = 0$$ may have equal roots.

Theory of Equations

If the roots of the equation $$x^{3} - ax^{2} + x - b = 0$$ are in harmonical progression, show that the mean root is $$3b$$.

Theory of Equations

$$2$$ and $$-2$$ are two zeros of the polynomial $$m^4+m^3-34m^{2} - 4m + 120$$. What are the other two zeros of the polynomial?

Contact Details