HELPPP MEE please

Cube both sides (inequality stays the same). Expand. Do some algebra to get something obviously true. The proof is your work, written backwards.
 
I did what you said
and i found 8a^3-15a²+6ab²+b^3>=0
but i don't know what to do :(
 
Really? For a second I thought I misspoke when I tried to do it myself. Good on you if you found it.

Here is a shorter way if you know the arithmetic-geometric mean inequality:

\(\displaystyle \displaystyle \frac{a_1+a_2+\cdots +a_n}{n} \ge \sqrt[n]{a_1a_2\cdots a_n}\)

Then

\(\displaystyle \displaystyle \frac{2a+b}{3} = \frac{a+a+b}{3} \ge \sqrt[3]{a\cdot a\cdot b} = \sqrt[3]{a^2b}\)
 
really? For a second i thought i misspoke when i tried to do it myself. Good on you if you found it.

Here is a shorter way if you know the arithmetic-geometric mean inequality:

\(\displaystyle \displaystyle \frac{a_1+a_2+\cdots +a_n}{n} \ge \sqrt[n]{a_1a_2\cdots a_n}\)

then

\(\displaystyle \displaystyle \frac{2a+b}{3} = \frac{a+a+b}{3} \ge \sqrt[3]{a\cdot a\cdot b} = \sqrt[3]{a^2b}\)

yeah this is a very good way thank you so much and i m sorry for disturbance

can i ask another question?
 
yeah this is a very good way thank you so much and i m sorry for disturbance

can i ask another question?

Yes, but in the future please post your attempt at a solution and state where you may be stuck
 
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