回復 8# ilikemath 的帖子
計算1.
設曲線\( y=f(x) \)(\( x \ge 0 \))過點\((0,0)\),且對於任意\(a>0\),此曲線在\(x=0\)與\(x=a\)間的弧長為\( \displaystyle \frac{2}{3}\Bigg[\; (1+a)^{\frac{3}{2}}-1 \Bigg]\; \)。若對於所有\( x \ge 0 \),都有\( f'(x) \le 0 \),則\( f(x)= \)?
[解答]
曲線長 \( = \int_0^a \sqrt{1+(f'(x))^2} dx\),對 a 微分,用微積分基本定理得
\( (f'(a))^2 = a \),又 \( f'(a)\leq 0 \) 且 \( f(0) = 0 \) 可得 \( f(x) = -\frac{2}{3}x^{\frac{3}{2}} \)