Looks tough! [part 2]

Algebra Level 4

If a , b , c a,b,c and d d are positive reals satisfying a + b + c + d = 20 a + b + c + d =20 , then find the maximum value of a b 2 c 3 d 4 ab^2c^3d^4 .

2 22 × 2 4 2^{22}\times 2^4 None of these choices 2 20 × 3 3 2^{20}\times 3^3 2 21 × 3 4 2^{21}\times 3^4

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1 solution

Rishabh Jain
Feb 17, 2016

I'll certainly generalise this :-).
Let a + b + c + d = M a+b+c+d=\mathfrak{M} and we have to find maximum value of K = a p b q c r d s \mathfrak{K}=a^{p}b^{q}c^{r}d^{s} .

Writing a = a p + a p p times , b = b q + b q q times , c = c r + c r r times , d = d s + d s s times a=\frac{a}{p}+\frac{a}{p}\cdots\text{p times},\\ b=\frac{b}{q}+\frac{b}{q}\cdots\text{q times},\\ c=\frac{c}{r}+\frac{c}{r}\cdots\text{r times},\\d=\frac{d}{s}+\frac{d}{s}\cdots\text{s times}\\ and applying AM-GM. ( K p p q q r r s s ) 1 p + q + r + s M p + q + r + s \Large(\dfrac{\mathfrak{K}}{p^pq^qr^rs^s})^{\frac{1}{p+q+r+s}}\leq\dfrac{\mathfrak{M}}{p+q+r+s} K ( M p + q + r + s ) p + q + r + s × p p q q r r s s \Large \Rightarrow \mathfrak{K}\leq(\dfrac{\mathfrak{M}}{p+q+r+s})^{p+q+r+s}\times p^pq^qr^rs^s For equality a p = b q = c r = d s = M p + q + r + s \color{#3D99F6}{\dfrac{a}{p}=\dfrac{b}{q}=\dfrac{c}{r}=\dfrac{d}{s}=\dfrac{\mathfrak{M}}{p+q+r+s}} .
In this question p = 1 , q = 2 , r = 3 , s = 4 \color{#D61F06}{p=1,q=2,r=3,s=4} and M = 20 \color{#D61F06}{\mathfrak{M}=20} . Substituting values we get : K 2 20 × 3 3 \huge \mathfrak{K}\leq \color{#007fff}{2^{20}\times 3^3} (In this question, for equality a = b 2 = c 3 = d 4 = 2 \color{#3D99F6}{ a=\frac{b}{2}=\frac{c}{3}=\frac{d}{4}=2} )

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