$(x – 100) (x – 102) (x – 104)\cdots(x – 200) > 0$ for how many natural $x<250$

algebra-precalculusinequalityproducts

For how many natural numbers $x$ less than $250$ is the inequality: $$(x – 100) (x – 102) (x – 104) (x – 106)\cdots(x – 200) > 0$$ satisfied?

I tried to plot general points to get an idea of the problem, and could easily see the first positive point would be beyond x > 200, and subsequent sign changes will happen along all the points coming backwards. How do we compute all those ranges without actually manually plotting all 200 points on a number line, and seeing between what values of x the sign of inequality is positive?

Best Answer

There are $51$ zeros of the polynomial, leaving $52$ continuous segments where the polynomial is either always positive or always negative. Hence there are $26$ positive and negative segments each; the rightmost such segment must be positive since the leading coefficient is positive, and contains $249-201+1=49$ numbers. The other $25$ positive segments each contain a single integer each, as is easy to see; the polynomial's degree is odd, so the large leftmost segment is negative. This gives the final answer as $49+25=74$.