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Why did the first book of ninth grade mathematics published by Beijing Normal University simplify (8-2x)(5-2x)= 18 to 2x2-13x+1= 0?
(8-2x)(5-2x)= 18

Expand the left side first, 40-16x-10x+4x2 =18.

Finishing, 22-26x+4x 2 = 0

Divide both sides by (-2) to get-1+13x+2x2 = 0.

Exchange sequence: 2x2-13x+11= 0.