300 Followers question!

Algebra Level 3

300 x 299 + 299 x 4 + 343 x 3 + 23 x + 300 = 0 300x^{299}+299x^4+343x^3+23x+300=0

If α \alpha is the real root of the equation above, find α \lfloor \alpha \rfloor .


The answer is -1.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

3 solutions

Nihar Mahajan
Apr 3, 2015

We can do this by eradicating cases.

If α > 0 \alpha>0 we see that whole expression always remains positive.

If α < 1 \alpha<-1 we see that 300 x 299 300x^{299} becomes too much large for the expression to be zero , making the polynomial always negative.

So we can easily conclude that 1 < α < 0 -1<\alpha<0 and hence α = 1 \lfloor \alpha \rfloor = -1

Let f ( x ) = 300 x 299 + 299 x 4 + 343 x 3 + 23 x + 300 f(x) = 300x^{299}+299x^4+343x^3+23x+300 . We note that: f ( 0 ) = 300 f(0) = 300 and f ( 1 ) = 67 f(-1) = -67 . Therefore, there is a real root of f ( x ) f(x) in 1 < x < 0 -1<x<0 . And this is the only root because for x < 2 x<-2 or x > 2 x>2 , f ( x ) 300 x 299 f(x) \approx 300x^{299} a constantly increasing function. Therefore, 1 < α < 0 α = 1 -1< \alpha < 0\quad \Rightarrow \lfloor \alpha \rfloor = \boxed{-1} .

The following graph shows the real root α \alpha .

Deepak Kumar
Apr 3, 2015

By observation, it is clear that the root is negative as the expression is positive for x>0 and equals 300 at x=0.Now by some hit and trial f(-1)<0 => f(0)*f(-1)<0=> The real root lies between -1 and 0 => [alpha]=-1

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...