Divide the number 4 into two positive number such that the sum of the square of one and the cube of other is minimum

Dear Student,
Please find below the solution to the asked query:

$$\begin{gathered} \mathrm{Let} \mathrm{two} \mathrm{numbers} \mathrm{be} \mathrm{x} \mathrm{and} \mathrm{y}. \\ \mathrm{We} \mathrm{have} \\ \mathrm{x}+\mathrm{y}=4 \\ \Rightarrow \mathrm{y}=4-\mathrm{x} \\ \mathrm{Let} \\ \mathrm{S}={\mathrm{x}}^3+{\mathrm{y}}^2 \\ ={\mathrm{x}}^3+{\left(4-\mathrm{x}\right)}^2 \\ \mathrm{S}={\mathrm{x}}^3+{\mathrm{x}}^2-8\mathrm{x}+16 \\ \Rightarrow \frac{\mathrm{dS}}{\mathrm{dx}}=3{\mathrm{x}}^2+2\mathrm{x}-8 \\ \mathrm{For} \mathrm{maxima} \mathrm{or} \mathrm{minima} \frac{\mathrm{dS}}{\mathrm{dx}}=0 \\ \Rightarrow 3{\mathrm{x}}^2+2\mathrm{x}-8=0 \\ \Rightarrow 3{\mathrm{x}}^2+6\mathrm{x}-4\mathrm{x}-8=0 \\ \Rightarrow 3\mathrm{x}\left(\mathrm{x}+2\right)-4\left(\mathrm{x}+2\right)=0 \\ \Rightarrow \left(3\mathrm{x}-4\right)\left(\mathrm{x}+2\right)=0 \\ \mathrm{Either} 3\mathrm{x}-4=0 \mathrm{or} \mathrm{x}+2=0 \\ \Rightarrow \mathrm{x}=\frac{4}{3} \mathrm{or} \mathrm{x}=-2 \\ \frac{\mathrm{dS}}{\mathrm{dx}}=3{\mathrm{x}}^2+2\mathrm{x}-8 \\ \Rightarrow \frac{{\mathrm{d}}^2\mathrm{S}}{{\mathrm{dx}}^2}=6\mathrm{x}+2 \\ \mathrm{Putting} \mathrm{x}=\frac{4}{3}, \mathrm{we} \mathrm{get}, \\ \frac{{\mathrm{d}}^2\mathrm{S}}{{\mathrm{dx}}^2}=6.\frac{4}{3}+2=14>0 \\ \mathrm{As} \frac{{\mathrm{d}}^2\mathrm{S}}{{\mathrm{dx}}^2}>0 \mathrm{at} \mathrm{x}=\frac{4}{3} \\ \Rightarrow \mathrm{S} \mathrm{will} \mathrm{ne} \mathrm{minimum} \mathrm{when} \mathbf{x}\mathbf{=}\frac{\mathbf{4}}{\mathbf{3}} \\ \mathrm{y}=4-\frac{4}{3}=\frac{12-4}{3} \\ \mathbf{\Rightarrow }\mathbf{y}\mathbf{=}\frac{\mathbf{8}}{\mathbf{3}} \\ \mathbf{Hence}\mathbf{ }\mathbf{numbers}\mathbf{ }\mathbf{are}\mathbf{ }\frac{\mathbf{4}}{\mathbf{3}}\mathbf{ }\mathbf{and}\mathbf{ }\frac{\mathbf{8}}{\mathbf{3}}\mathbf{.} \end{gathered}$$

Hope this information will clear your doubts about this topic.

If you have any doubts just ask here on the ask and answer forum and our experts will try to help you out as soon as possible.
Regards