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20 10 the answer to the third question of the second liberal arts in the national college entrance examination mathematics volume
(3) given sina=2/3, cos(π-2a)= 1
(A) (B) (C) (D)
Analysis: cos (π-2a) =-cos2a =-(1-2sin2a) = 2sin2a-1= 2 * 4/9-1=-19, choose B.