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# hellllppppp????

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Given that $$xy = \dfrac32$$ and both x and y are nonnegative real numbers, find the minimum value of $$10x + \dfrac{3y}5.$$

Jan 27, 2018

#1
+100800
+2

Given that  $$xy = \dfrac32$$   and both x and y are nonnegative real numbers, find the minimum value of

$$10x + \dfrac{3y}5$$

$$y=\frac{3}{2x}$$

so

$$​​​​z=10x+\frac{3y}{5}\\ ​​​​z=10x+\frac{3}{5}\times \frac{3}{2x}\\ ​​​​z=10x+\frac{9}{10x}\\ ​​​​z=10x+0.9x^{-1}\\ \frac{dz}{dx}=10-0.9x^{-2}\\ \frac{d^2z}{dx^2}=1.8x^{-3}>0\qquad \text{So any stationary point will be a minimum.}\\ \text{Find stat point}\\ 10-0.9x^{-2}=0\\ 10=0.9x^{-2}\\ 10=\frac{9}{10x^2}\\ 100x^2=9\\ x^2=\frac{9}{100}\\ x=\frac{3}{10}\qquad \text{Since x>0}\\ so\\ ​​​​z=10x+0.9x^{-1}\\ min\;\;value\\ =10*\frac{3}{10}+\frac{9*10}{10*3}\\ =3+3\\ =6$$

check:

Here is the graph:

Jan 28, 2018

#1
+100800
+2

Given that  $$xy = \dfrac32$$   and both x and y are nonnegative real numbers, find the minimum value of

$$10x + \dfrac{3y}5$$

$$y=\frac{3}{2x}$$

so

$$​​​​z=10x+\frac{3y}{5}\\ ​​​​z=10x+\frac{3}{5}\times \frac{3}{2x}\\ ​​​​z=10x+\frac{9}{10x}\\ ​​​​z=10x+0.9x^{-1}\\ \frac{dz}{dx}=10-0.9x^{-2}\\ \frac{d^2z}{dx^2}=1.8x^{-3}>0\qquad \text{So any stationary point will be a minimum.}\\ \text{Find stat point}\\ 10-0.9x^{-2}=0\\ 10=0.9x^{-2}\\ 10=\frac{9}{10x^2}\\ 100x^2=9\\ x^2=\frac{9}{100}\\ x=\frac{3}{10}\qquad \text{Since x>0}\\ so\\ ​​​​z=10x+0.9x^{-1}\\ min\;\;value\\ =10*\frac{3}{10}+\frac{9*10}{10*3}\\ =3+3\\ =6$$

check:

Here is the graph:

Melody Jan 28, 2018