2x^2-28x=-56 x^2-14x=-28 x^2-14x+28=0 (x-7)^2=21 x-7=±√21 ∴X1=√21+7 X2=-√21+7
原式化简(同除以2)x^2-14x+28=0由于判别式14^-4*28=84x=14+2√21/2或14-2√21/2即x=√21+7或=-√21+7