#NEWLINE# def test_straight_line( ) : #NEWLINE# #INDENT# F = f ( x ) #NEWLINE# Fd = F . diff ( x ) #NEWLINE# L = sqrt ( ( 1 + ( Fd ** 2 ) ) ) #NEWLINE# assert ( diff ( L , F ) == 0 ) #NEWLINE# assert ( diff ( L , Fd ) == ( Fd / sqrt ( ( 1 + ( Fd ** 2 ) ) ) ) )