Two possible values of x satisfying sin x = 2 can be expressed as
b a π − ln ( c ± d ) i
where a , b , c , and d are positive integers, for which ∣ ∣ ∣ b a ∣ ∣ ∣ < 1 , with a and b being coprime and d square-free.
Find a + b + c + d .
Notations:
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sin x 2 i ( e − x i − e x i ) e − x i − e x i 1 − e 2 x i e 2 x i − 4 e x i i − 4 ( e x i − 2 i ) 2 e x i − 2 i e x i e x i i x ⟹ x = 2 = 2 = − 4 i = − 4 e x i i = 1 − 4 = − 3 = ± 3 i = ( 2 ± 3 ) i = ( 2 ± 3 ) e 2 π i = ln ( 2 ± 3 ) + 2 π i = 2 π − ln ( 2 ± 3 ) i Multiply both sides by e i x Take square root both sides Euler’s formula: e θ i = cos θ + i sin θ Take natural log both sides
⟹ a + b + c + d = 1 + 2 + 2 + 3 = 8
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Let x = A + B i such that s i n ( x ) = 2 i e i x − e − i x = 2 ⇒ e i x − e − i x = 4 i . Further substitution and simplification yields:
e i ( A + B i ) − e − i ( A + B i ) = e A i e B i 2 − e − A i e − B i 2 = e − B e A i − e B e − A i = e − B ( c o s ( A ) + i ⋅ s i n ( A ) ) − e B ( c o s ( A ) − i ⋅ s i n ( A ) ) = 4 i ;
which after matching the real and imaginary components gives:
e − B c o s ( A ) − e B c o s ( A ) = 0 and e − B s i n ( A ) + e B s i n ( A ) = 4
which produces A = 2 π and e − B + e B = 4 ⇒ e 2 B − 4 e B + 1 = 0 . Let u = e B to obtain the quadratic u 2 − 4 u + 1 = 0 ⇒ u = 2 4 ± 4 2 − 4 ( 1 ) ( 1 ) = 2 4 ± 1 2 = 2 4 ± 2 3 = 2 ± 3 , or B = l n ( 2 ± 3 ) .
Altogether, x = A + B i = 2 π + l n ( 2 ± 3 ) i , which leads to 1 + 2 + 2 + 3 = 8 .