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Parallel lines in the second volume of seventh grade mathematics
As shown in the figure, the extension line extending DE and BC intersects at point F, and AB is parallel to DE, then we know that angle EFB= angle ABF, angle EDC= angle EFB+ angle DCF and angle DCB= 180- angle DCF, then we know that angle EDC= 180- angle DCB+ angle ABC. That is, angle D= 180- angle DCB+ angle b.