∑ k = 1 9 0 cos k ∘ ∑ k = 1 9 0 sin k ∘ = − tan x ∘
If the equation above holds true for 0 ≤ x ≤ 1 8 0 , what is the value of x + 2 1 ?
Note: All are in degrees.
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
insert
product to sum
telescoping sum
sum to prod.
the rest is trivial.
∑ cos ( k A ) ∑ sin ( k A ) when A = 1 ∘ , n = 9 0 , tan ( 2 n + 1 A ) ∴ x + 2 1 = ∑ 2 sin ( 2 A ) cos ( k A ) ∑ 2 sin ( 2 A ) sin ( k A ) = ∑ [ sin ( k + 2 1 ) A − sin ( k − 2 1 ) A ] ∑ [ − cos ( k + 2 1 ) A + cos ( k − 2 1 ) A ] = sin ( n + 2 1 ) A − sin ( 1 − 2 1 ) A − cos ( n + 2 1 ) A + cos ( 1 − 2 1 ) A = 2 sin ( 2 n A ) cos ( 2 n + 1 A ) 2 sin ( 2 n A ) sin ( 2 n + 1 A ) = tan ( 2 n + 1 A ) = tan 4 5 . 5 ∘ = − tan 1 3 4 . 5 ∘ = 1 3 5
Q = ∑ k = 1 9 0 cos k ∘ ∑ k = 1 9 0 sin k ∘ = ∑ k = 1 9 0 cos k ∘ ∑ k = 1 9 0 cos ( 9 0 − k ) ∘ = ∑ k = 1 8 9 cos k ∘ ∑ k = 0 8 9 cos k ∘ = ∑ k = 1 8 9 cos k ∘ cos 0 ∘ + ∑ k = 1 8 9 cos k ∘ = ∑ k = 1 8 9 cos k ∘ 1 + 1 = ∑ k = 1 8 9 ℜ ( e k ∘ i ) 1 + 1 = ℜ ( ∑ k = 1 8 9 e k ∘ i ) 1 + 1 = ℜ ( 1 − e 1 ∘ i e 1 ∘ i − e 9 0 ∘ i ) 1 + 1 = [ ℜ ( 1 − e 1 ∘ i e 1 ∘ i − i ) ] − 1 + 1 = [ ℜ ( 1 − cos 1 ∘ − i sin 1 ∘ cos 1 ∘ + i sin 1 ∘ − i ) ] − 1 + 1 = [ ℜ ( 1 − 1 + t 2 1 − t 2 − 1 + t 2 2 t i 1 + t 2 1 − t 2 + 1 + t 2 2 t i − i ) ] − 1 + 1 = [ ℜ ( 2 t 2 − 2 t i 1 − t 2 − i ( 1 − t ) 2 ) ] − 1 + 1 = [ ℜ ( 2 t ( t − i ) ( 1 − t ) ( 1 + t − i ( 1 − t ) ) ] − 1 + 1 = [ ℜ ( 2 t ( t − i ) ( t + i ) ( 1 − t ) ( 1 + t − i ( 1 − t ) ) ( t + i ) ) ] − 1 + 1 = [ ℜ ( 2 t ( t 2 + 1 ) ( 1 − t ) ( t 2 + 1 + i ( 1 + t 2 ) ) ) ] − 1 + 1 = 1 − t 2 t + 1 = 1 − t 1 + t = 1 − tan 4 5 ∘ tan 0 . 5 ∘ tan 4 5 ∘ + tan 0 . 5 ∘ = tan 4 5 . 5 ∘ = − tan 1 3 4 . 5 ∘ Since sin θ = cos ( 9 0 ∘ − θ ) And cos 9 0 ∘ = 0 By Euler’s formula: e θ i = cos θ + i sin θ where ℜ ( ⋅ ) is the real part of a complex number. Let t = tan 0 . 5 ∘
Since Q = − tan x , ⟹ x = 1 3 4 . 5 and x + 2 1 = 1 3 5 .
Problem Loading...
Note Loading...
Set Loading...
Take note that tan 4 5 = 1 .
x = 1 ∑ 9 0 sin x = 1 + x = 0 ∑ 8 9 sin x = 1 + Im ( x = 0 ∑ 8 9 ( cis 1 ) x ) = 1 + Im ( ( cis 1 ) − 1 ( cis 1 ) 9 0 − ( cis 1 ) 0 ) = 1 + Im ( ( cis 1 ) − 1 ( cis 9 0 ) − 1 ) = 1 + Im ( ( ( cos 1 ) − 1 ) + i ( sin 1 ) i − 1 ⋅ ( ( cos 1 ) − 1 ) − i ( sin 1 ) ( ( cos 1 ) − 1 ) − i ( sin 1 ) ) = 1 + Im ( 2 − 2 cos 1 ( ( sin 1 ) − ( cos 1 ) + 1 ) + i ( ( sin 1 ) + ( cos 1 ) − 1 ) ) = 1 + ( 2 1 [ 1 − cos 1 − ( 1 − cos 1 ) + ( sin 1 ) ] ) = 1 + ( 2 1 [ 1 − cos 1 − ( 1 − cos 1 ) + 1 − cos 1 ( sin 1 ) ] ) = 1 + ( 2 1 [ − 1 + tan ( 2 1 7 9 ) ] ) = 2 tan 4 5 + tan ( 2 1 7 9 )
and,
x = 1 ∑ 9 0 cos x = − 1 + x = 0 ∑ 8 9 cos x = − 1 + Re ( x = 0 ∑ 8 9 ( cis 1 ) x ) = − 1 + Re ( ( cis 1 ) − 1 ( cis 1 ) 9 0 − ( cis 1 ) 0 ) = − 1 + Re ( ( cis 1 ) − 1 ( cis 9 0 ) − 1 ) = − 1 + Re ( ( ( cos 1 ) − 1 ) + i ( sin 1 ) i − 1 ⋅ ( ( cos 1 ) − 1 ) − i ( sin 1 ) ( ( cos 1 ) − 1 ) − i ( sin 1 ) ) = − 1 + Re ( 2 − 2 cos 1 ( ( sin 1 ) − ( cos 1 ) + 1 ) + i ( ( sin 1 ) + ( cos 1 ) − 1 ) ) = − 1 + ( 2 1 [ 1 − cos 1 ( 1 − cos 1 ) + ( sin 1 ) ] ) = − 1 + ( 2 1 [ 1 − cos 1 ( 1 − cos 1 ) + 1 − cos 1 ( sin 1 ) ] ) = − 1 + ( 2 1 [ 1 + tan ( 2 1 7 9 ) ] ) = − 2 1 − [ ( tan 4 5 ) ( tan ( 2 1 7 9 ) ) ]
Then, the above expression is simply equal to:
− 1 − [ ( tan 4 5 ) ( tan ( 2 1 7 9 ) ) ] tan 4 5 + tan ( 2 1 7 9 ) = − tan ( 2 1 7 9 + 4 5 ) = − tan ( 1 3 4 . 5 ) ∴ x + 2 1 = 1 3 5
Additional note:
Using the half - tangent formula tan ( 2 x ) = 1 + cos x sin x ,
1 − cos 1 sin 1 = 1 + cos 1 7 9 sin 1 7 9 = tan ( 2 1 7 9 ) .