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∵a+c=2b∴sinA+sinC=2sinB,,即2sinA+C2cosA-C2=4sinB2cosB2,∴sinB2=12cosA-C2=34,而0<;B2<;π2,∴cosB2=134,∴sinB=2sinB2cosB2=2×34×134=398.
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