t( 15 ) = t( 1 ) + [ 14 × ( Constant value ) ]
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t( 1 ) = 1
&
Constant value = t( 2 ) - t( 1 ) = t( 3 ) - t( 2 )
Constant value = 6 - 1
Constant value = 5
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Thus :
t ( 15 ) = 1 + [ 14 × 5 ]
t( 15 ) = 1 + 70
t( 15 ) = 71