在三角形ABC中 a+c=2b ∠A-∠C=60° 则sinB=?sinA + sinC = 2sinB=2sin(A+C)2sin[(A+C)/2] * cos[(A-C)/2] = 2 * 2 * sin(B/2) * cos(B/2)这两个式子怎么列出来的?
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