cos2A-cos2C=2sinAsinC = -2sin(A+C)sin(A-C) , a,b,c依次成等比数列,b^2 = ac , (sinB)^2 = sinA * sinC , 所以 -sinB = sin(A-C) , 即sin(C-A) = sin(C+A) , 所以cosC * sinA = 0 , C = 90° , ABC是直角三角形。由cosB=3/4 ,向量BA与向量BC的模的比值 = 4:3 ,由数量积为3/2 , 解得/BC/ = (根6)/2 , /BA/ = 2(根6)/3 , a+c = 7(根6)/6... |