# If a, b and c are positive real numbers, prove that 4(a³ + b³) ≥ (a + b)³

• Oct 1st 2009, 04:50 AM
Mozart
If a, b and c are positive real numbers, prove that 4(a³ + b³) ≥ (a + b)³
If a, b and c are positive real numbers, prove that 4(a³ + b³) ≥ (a + b)³ & that 9(a³ + b³ + c³) ≥ (a + b + c)³
• Oct 1st 2009, 05:58 AM
bram kierkels
Quote:

Originally Posted by Mozart
If a, b and c are positive real numbers, prove that 4(a³ + b³) ≥ (a + b)³ & that 9(a³ + b³ + c³) ≥ (a + b + c)³

for the first one;
\$\displaystyle 4a^3+4b^3\geq a^3+3a^2b+3ab^2+b^3\$
\$\displaystyle \Longleftrightarrow 3a^3+3b^3\geq 3a^2b+3ab^2\$
\$\displaystyle \Longleftrightarrow a^3+b^3\geq a^2b+ab^2\$
\$\displaystyle \Longleftrightarrow (a+b)(a^2-ab+b^2)\geq (a+b)(ab)\$
\$\displaystyle \Longleftrightarrow a^2-ab+b^2\geq ab \$
\$\displaystyle \Longleftrightarrow a^2-2ab+b^2\geq 0\$
\$\displaystyle \Longleftrightarrow (a-b)^2\geq 0\$
• Oct 1st 2009, 11:58 AM
Krizalid
that's wrong proof.

to fix it, in each step you need to add an \$\displaystyle \iff.\$