Find the value of x satisfying the equation below.
( 2 ! 2 0 1 6 π 2 ) ( 3 ! 2 0 1 6 π 3 ) ( 6 ! 2 0 1 6 π 6 ) ( 7 ! 2 0 1 6 π 7 ) ( 1 0 ! 2 0 1 6 π 1 0 ) ( 1 1 ! 2 0 1 6 π 1 1 ) ( … ) ( 0 ! 2 0 1 6 π 0 ) ( 1 ! 2 0 1 6 π 1 ) ( 4 ! 2 0 1 6 π 4 ) ( 5 ! 2 0 1 6 π 5 ) ( 8 ! 2 0 1 6 π 8 ) ( 9 ! 2 0 1 6 π 9 ) ( … ) = 2 0 1 6 x
Notation : n ! denotes the factorial function. For example, 8 ! = 1 × 2 × 3 × ⋯ × 8 .
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Rewrite the above expression :
( 2 ! 2 0 1 6 π 2 ) ( 3 ! 2 0 1 6 π 3 ) ( 6 ! 2 0 1 6 π 6 ) ( 7 ! 2 0 1 6 π 7 ) ( 1 0 ! 2 0 1 6 π 1 0 ) ( 1 1 ! 2 0 1 6 π 1 1 ) ( … ) ( 0 ! 2 0 1 6 π 0 ) ( 1 ! 2 0 1 6 π 1 ) ( 4 ! 2 0 1 6 π 4 ) ( 5 ! 2 0 1 6 π 5 ) ( 8 ! 2 0 1 6 π 8 ) ( 9 ! 2 0 1 6 π 9 ) ( … )
= exp 2 0 1 6 ( 1 + π − 2 ! π 2 − 3 ! π 3 + 4 ! π 4 + 5 ! π 5 − 6 ! π 6 − 7 ! π 7 + … )
= exp 2 0 1 6 ( sin ( π ) + cos ( π ) )
= exp 2 0 1 6 ( − 1 )
⇒ x = − 1
Notation : exp A ( B ) denotes the exponential function A B .
Consider the power series for cos ( x ) and sin ( x ) :
sin ( x ) = x − 3 ! x 3 + 5 ! x 5 − 7 ! x 7 + …
sin ( π ) = π − 3 ! π 3 + 5 ! π 5 − 7 ! π 7 + …
cos ( x ) = 1 − 2 ! x 2 + 4 ! x 4 − 6 ! x 6 + …
cos ( π ) = 1 − 2 ! π 2 + 4 ! π 4 − 6 ! π 6 + …
sin ( π ) + cos ( π ) = 1 + π − 2 ! π 2 − 3 ! π 3 + 4 ! π 4 + 5 ! π 5 − 6 ! π 6 − 7 ! π 7 + …
sin ( π ) + cos ( π ) = 0 − 1 = − 1