Subjective Type

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

Solution

Given equation, $$x^4-12x^2+12x-3=0$$

Consider $$f(x) =x^4-12x^2+12x-3$$.

Then $$f(-3) = -66$$ and $$f(-4) = 13$$

Since the signs of $$f(-3)$$ and $$f(-4)$$ are opposite, $$f(x)$$ must cross x-axis atleast once in the interval $$(-4,-3)$$.

$$\therefore\,\, f(x) = 0$$ must have one root between $$-3$$ and $$-4$$.

Similarly, $$f(2) = -11$$ and $$f(3) = 6$$.
Since the signs of $$f(2)$$ and $$f(3)$$ are different, $$f(x) = 0$$ must have one root between $$2$$ and $$3$$.


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