sin ( ln x ) + 2 cos ( 3 ln x ) sin ( 2 ln x ) = 0
Find the smallest x > 1 satisfying the equation above.
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Let θ = ln x . Then we have:
sin θ + 2 cos 3 θ sin 2 θ sin θ + 2 ( 4 cos 3 θ − 3 cos θ ) ⋅ 2 sin θ cos θ 1 6 cos 4 θ − 1 2 cos 2 θ + 1 cos 2 θ cos θ θ = ln x = 0 = 0 = 0 = 8 3 ± 5 = ± 4 5 ± 1 = ( n + 5 k ) π Since θ = ln x > 0 ⟹ sin θ = 0 Divide both sides by sin θ where n ∈ Z , k = 1 , 2 , 3 , 4
For the smallest x > 1 , ⟹ θ > ln 1 = 0 or ( n + 5 k ) > 0 ⟹ n = 0 , k = 1 and ln x = 5 π ⟹ x = e 5 π ≈ 1 . 8 7 .
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s i n ( l o g ( x ) ) + 2 c o s ( 3 l o g ( x ) ) s i n ( 2 l o g ( x ) ) = s i n ( l o g ( x ) ) + s i n ( 5 l o g ( x ) ) − s i n ( l o g ( x ) ) = s i n ( 5 l o g ( x ) ) = 0 ⟹ 5 l o g ( x ) = k π where ( k ∈ Z ) x = e 5 k π Then solution is x = e 5 π = 1 . 8 7 4 4 Here smallest possible n = 1 [as x > 1 and x = 0 ]