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If ϕ = n + n + n + . . . \phi = \sqrt {n + \sqrt {n + \sqrt {n + ...}}} , where ϕ \phi represents 1 + 5 2 \frac {1 + \sqrt {5}}{2} , and n + n i 3 n n i \frac {n + ni}{3n - ni} is a complex number which can be solved to equal a + b i c \frac {a + bi}{c} , where i 2 i^2 equals 1 -1 and a a , b b and c c are positive integers, find the value of ( a + b + c ) c a + b (a + b + c)^{\frac {c}{a + b}} ?


The answer is 32.

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4 solutions

Lorenc Bushi
Jan 15, 2014

If we square the given expression we get:

ϕ 2 = n + n + n + . . . . . \phi^2=n+\sqrt{n+\sqrt{n+.....}} .Take n n to the left side and we have:

ϕ 2 n = n + n + . . . . . \phi^2-n=\sqrt{n+\sqrt{n+.....}} .But n + n + . . . . = ϕ \sqrt{n+\sqrt{n+....}}=\phi , therefore our equation transforms into :

ϕ 2 n = ϕ \phi^2-n=\phi where ϕ 2 = ϕ + n \phi^2=\phi+n .We know that ϕ \phi satisfies the equation ϕ 2 = ϕ + 1 \phi^2=\phi+1 and equating this to our equation we easily get n = 1 \boxed{n=1} .From this we know our complex number is :

1 + i 3 i \dfrac{1+i}{3-i} .Multiplying both the numerator and the denominator by 3 + i 3+i our complex number bcomes :

( 1 + i ) ( 3 + i ) ( 3 i ) ( 3 + i ) = 4 i + 2 10 = 2 i + 1 5 \dfrac{(1+i)(3+i)}{(3-i)(3+i)}=\dfrac{4i+2}{10}=\dfrac{2i+1}{5} .Thus a = 1 \boxed{a=1} , b = 2 \boxed{b=2} and c = 5 \boxed{c=5} .And we have ( a + b + c ) c a + b = 8 5 3 = 32 (a+b+c)^{\dfrac{c}{a+b}}=8^{\dfrac{5}{3}}=\boxed{32} .

What a nice final expression.

It is "quite known" that φ = 1 + 1 + \varphi = \sqrt{1+\sqrt{1+\sqrt{\cdots}}} . Thus n = 1 n =1 .

Simplifying 1 + i 3 i \frac{1+i}{3-i} , we have ( 1 + i ) ( 3 + i ) ( 3 i ) ( 3 + i ) = a + b i c 2 + 4 i 10 = a + b i c \frac{(1+i)(3+i)}{(3-i)(3+i)} = \frac{a+bi}{c} \Rightarrow \frac{2+4i}{10} = \frac{a+bi}{c} and thus a = 1 , b = 2 , c = 5 a = 1, b = 2, c= 5 .

For it the final expression becomes ( 1 + 2 + 5 ) 5 1 + 2 2 3 5 3 32. (1+2+5)^{\frac{5}{1+2}} \Rightarrow 2^{3 \cdot \frac{5}{3}} \Rightarrow \boxed{32.}

Note that n n is non-zero, so n + n i 3 n n i \dfrac{n + ni}{3n - ni} automatically simplifies to 1 + i 3 i \dfrac{1 + i}{3 - i} .

Jon Haussmann - 7 years, 4 months ago
Prasun Biswas
Feb 9, 2014

Given that ϕ = 1 + 5 2 \phi=\frac{1+\sqrt{5}}{2} and n + n i 3 n n i \frac{n+ni}{3n-ni} is a complex no. which can be solved down to a + b i c \frac{a+bi}{c} . So, at first we have ---

ϕ = n + n + n + . . . . \phi = \sqrt{n+\sqrt{n+\sqrt{n+....}}} .....(i)

ϕ 2 = n + n + n + n + . . . . \phi^2=n+ \sqrt{n+\sqrt{n+\sqrt{n+....}}} [Squaring both sides]

ϕ 2 = n + ϕ \phi^2=n+ \phi [From (i)]

( 1 + 5 2 ) 2 = n + ( 1 + 5 2 ) (\frac{1+\sqrt{5}}{2})^2=n+(\frac{1+\sqrt{5}}{2})

1 + 2 5 + 5 4 = 2 n + 1 + 5 2 \frac{1+2\sqrt{5}+5}{4}=\frac{2n+1+\sqrt{5}}{2}

2 5 + 6 2 = 2 n + 1 + 5 4 n + 2 + 2 5 = 6 + 2 5 4 n = 4 n = 4 \frac{2\sqrt{5}+6}{2}=2n+1+\sqrt{5} \implies 4n+2+2\sqrt{5}=6+2\sqrt{5} \implies 4n=4 \implies n=\boxed{4}

Then, the complex no. in the question is n + n i 3 n n i = 1 + i 3 i \frac{n+ni}{3n-ni}=\frac{1+i}{3-i} . Then, we solve this as follows --

1 + i 3 i \frac{1+i}{3-i}

= ( 1 + i ) ( 3 + i ) ( 3 i ) ( 3 + i ) =\frac{(1+i)(3+i)}{(3-i)(3+i)}

= 3 + i + 3 i + i 2 3 2 i 2 =\frac{3+i+3i+i^2}{3^2-i^2} [Using the identity a 2 b 2 = ( a + b ) ( a b ) a^2-b^2=(a+b)(a-b) ]

= 3 + 4 i 1 9 + 1 =\frac{3+4i-1}{9+1} [since i 2 = ( 1 ) i^2=(-1) ]

= 2 + 4 i 10 = 2 ( 1 + 2 i ) 10 = 1 + 2 i 5 =\frac{2+4i}{10} = \frac{2(1+2i)}{10} = \frac{1+2i}{5}

So, we have the complex no. in a + b i c \frac{a+bi}{c} form with a = 1 , b = 2 , c = 5 a=1,b=2,c=5 . Then, we have ---

( a + b + c ) c a + b = ( 1 + 2 + 5 ) 5 1 + 2 = ( 8 ) 5 3 = ( 2 3 ) 5 3 = ( 2 ) 3 × 5 3 = 2 5 = 32 (a+b+c)^{\frac{c}{a+b}}=(1+2+5)^{\frac{5}{1+2}} = (8)^{\frac{5}{3}} = (2^3)^{\frac{5}{3}} = (2)^{3\times \frac{5}{3}} = 2^5= \boxed{32}

ϕ = s q r t ( n + s q r t ( n + . . . ) ) \phi = sqrt(n + sqrt(n + ...))

Thus, ϕ = s q r t ( n + ϕ ) \phi = sqrt(n + \phi) .

Squaring both sides: ϕ 2 = n + ϕ \phi^{2} = n + \phi .

Since ϕ 2 = ϕ + 1 \phi^{2} = \phi + 1 , we can conclude that n = 1 n = 1 .

Our complex number is then ( 1 + i ) / ( 3 i ) (1 + i)/(3 - i) . Multiplying this by ( 3 + i ) / ( 3 + i ) (3 + i)/(3 + i) yields: ( 2 + 4 i ) / ( 10 ) = ( 1 + 2 i ) / 5 (2 + 4i)/(10) = (1 + 2i)/5 . You should have been more careful when you stated that the complex number can be represented as ( a + b i ) / c (a + bi)/c , since there are infinite representations that are of the form ( n a + n b i ) / n c (na + nbi)/nc , n n a nonzero real number, which equal the complex number you gave us - and those will yield different results. I'll assume that you wanted us to consider a = 1, b = 2 and c = 5. Then the answer would be ( 1 + 2 + 5 ) ( 5 / ( 1 + 2 ) ) (1 + 2 + 5)^{(5/(1 + 2))} , which equals 8 5 / 3 8^{5/3} , or 2 5 2^{5} , which is 32 32

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