Complex Indeed!

Algebra Level 5

p = 1 32 ( 3 p + 2 ) [ q = 1 10 ( sin ( 2 π q 11 ) i cos ( 2 π q 11 ) ) ] p \displaystyle \large \sum_{p=1}^{32} (3p+2) \left [ \sum_{q=1}^{10} \left ( \sin \left ( \frac {2\pi q}{11} \right ) - i \cos \left ( \frac {2\pi q}{11} \right ) \right ) \right ]^p

I have a summation within a summation in the above expression. What is its value?

Details and Assumptions :

i i is an imaginary number such that i 2 = 1 i^2 = -1

More questions?

None of the given choices. 16 ( 1 i ) 16(1-i) 8 ( 1 i ) 8(1-i) 48 ( 1 i ) 48(1-i)

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1 solution

Anandhu Raj
Feb 11, 2015

First we take q = 1 10 ( s i n 2 q π 11 i c o s 2 q π 11 ) \sum _{ q=1 }^{ 10 }{ \left( sin\frac { 2q\pi }{ 11 } -icos\frac { 2q\pi }{ 11 } \right) }

= i q = 1 10 ( c o s 2 q π 11 + i s i n 2 q π 11 ) =-i\sum _{ q=1 }^{ 10 }{ \left( cos\frac { 2q\pi }{ 11 } +isin\frac { 2q\pi }{ 11 } \right) }

= i q = 1 10 ( e i 2 π 11 ) q =\quad -i\sum _{ q=1 }^{ 10 }{ { \left( { e }^{ i\frac { 2\pi }{ 11 } } \right) }^{ q } }

= i q = 1 10 α q , w h e r e α = e i 2 π 11 =-i\sum _{ q=1 }^{ 10 }{ { \alpha }^{ q } } \quad \quad ,where\quad \alpha ={ e }^{ i\frac { 2\pi }{ 11 } }

= i α ( 1 α 10 ) 1 α =-i\frac { \alpha (1-{ \alpha }^{ 10 }) }{ 1-\alpha }

= i ( α α 11 ) 1 α =-i\frac { (\alpha -{ \alpha }^{ 11 }) }{ 1-\alpha }

= i α 1 1 α b c z e i 2 π 11 × 11 = e i 2 π = 1 =-i\frac { \alpha -1 }{ 1-\alpha } \quad \quad \quad \quad bcz\quad { e }^{ i\frac { 2\pi }{ 11 } \times 11 }={ e }^{ i2\pi }=1

q = 1 10 ( s i n 2 q π 11 i c o s 2 q π 11 ) = i \Longrightarrow \sum _{ q=1 }^{ 10 }{ \left( sin\frac { 2q\pi }{ 11 } -icos\frac { 2q\pi }{ 11 } \right) } =i ( 1 ) ---------(1)

Now ,

p = 1 32 ( 3 p + 2 ) [ q = 1 10 ( s i n 2 q π 11 i c o s 2 q π 11 ) ] p \sum _{ p=1 }^{ 32 }{ (3p+2){ \left[ \sum _{ q=1 }^{ 10 }{ \left( sin\frac { 2q\pi }{ 11 } -icos\frac { 2q\pi }{ 11 } \right) } \right] }^{ p } }

= p = 1 32 ( 3 p + 2 ) i p , u s i n g ( 1 ) \sum _{ p=1 }^{ 32 }{ (3p+2){ i }^{ p } } \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad ,using\quad (1)

= 3 p = 1 32 p i p + 2 p = 1 32 i p = 3 A + 2 B ( 2 ) 3\sum _{ p=1 }^{ 32 }{ p{ i }^{ p } } +2\sum _{ p=1 }^{ 32 }{ { i }^{ p } } =3A+2B\quad \quad \quad \quad -----(2)

where A = i + 2 i 2 + 3 i 3 + . . . . . . + 32 i 32 \quad A=i+{ 2i }^{ 2 }+{ 3i }^{ 3 }+......+{ 32i }^{ 32 }

A i = i 2 + 2 i 3 + 3 i 4 + . . . . . . + 31 i 32 + 32 i 33 \therefore \quad Ai={ i }^{ 2 }+{ 2i }^{ 3 }+{ 3i }^{ 4 }+......+{ 31i }^{ 32 }+{ 32i }^{ 33 }

A ( 1 i ) = ( i + i 2 + i 3 + i 4 + . . . . . . + i 32 ) 32 i 33 \quad \therefore \quad A(1-i)=(i+{ i }^{ 2 }+{ i }^{ 3 }+{ i }^{ 4 }+......+{ i }^{ 32 })-{ 32i }^{ 33 }

A ( 1 i ) = 32 i \quad \quad \Longrightarrow A(1-i)=-32i

A = 32 i ( 1 i ) = 16 ( 1 i ) \Longrightarrow A=\frac { -32i }{ (1-i) } =16(1-i) ( 3 ) --------------(3)

B = i + i 2 + i 3 + i 4 + . . . . . . + i 32 = 0 \quad B=i+{ i }^{ 2 }+{ i }^{ 3 }+{ i }^{ 4 }+......+{ i }^{ 32 }=0 ( 4 ) ------------(4)

( 3 ) a n d ( 4 ) i n ( 2 ) \quad (3)\quad and\quad (4)in\quad (2)\Rightarrow

p = 1 32 ( 3 p + 2 ) [ q = 1 10 ( s i n 2 q π 11 i c o s 2 q π 11 ) ] p \sum _{ p=1 }^{ 32 }{ (3p+2){ \left[ \sum _{ q=1 }^{ 10 }{ \left( sin\frac { 2q\pi }{ 11 } -icos\frac { 2q\pi }{ 11 } \right) } \right] }^{ p } } = 48 ( 1 i ) \boxed{48(1-i)}

Nicely done :)

A Former Brilliant Member - 6 years, 3 months ago

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@Azhaghu Roopesh M Thanks :)

Anandhu Raj - 6 years, 3 months ago

gud one .Key here is taking i -i common.

shivamani patil - 6 years, 3 months ago

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