STEP II 1997 question 8 solution - The Student Room
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STEP II 1997 question 8 solution

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I'm not sure what the "standard explanation" is here. I think Siklos says you just need to draw a couple of vague wiggles and say the integral is the area under the curve. More formally, if f,g are cts, consider the integral of f-g, apply the fundamental theorem of calculus and the mean value theorem to show that \displaystyle \int_a^b (f-g)(x) dx = (b-a)[f(c)-g(c)] for some \displaystyle c \in (a,b) and so is greater than 0.

If p > q and x>=1 then \displaystyle x^p \ge x^q. So \displaystyle \int_1^x t^p dt \ge \int_1^x t^q dt. So \displaystyle \frac{x^p-1}{p} \ge \frac{x^q-1}{q} as desired.

If p > q and 0 <= x <1 then \displaystyle x^p \le x^q. So \displaystyle \int_x^1 t^p dt \le \int_x^1 t^q dt and so \displaystyle \frac{1-x^p}{p} \le \frac{1-x^q}{q}. Multiplying by -1 confirms the earlier inequality for x >=0.

Doing the same trick on the inequality we've just found gives us \displaystyle \int_0^x \frac{1}{p} (x^p - 1) dx \ge \int_0^x \frac{1}{q} (x^q - 1) dx and so

\displaystyle \frac{1}{p}\left(\frac{x^{p+1}} {p+1} - x \right) \ge \frac{1}{q}\left(\frac{x^{q+1}}{q+1} - x \right).

If x > 0, dividing by x gives the desired result, while if x=0 the result is obviously true.

Solution by DFranklin

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