ab + cd = m*(a-c), ab + cd = am - cm, am - cm - ab - cd = 0, добавим к обеим частям (ad + bc), ad + bc = (am - cm) - ab -cd + ad + bc = m*(a-c) + (bc - ab) + (ad - cd) = m*(a-c) - b*(a-c) + d*(a-c) = [ (a-c) выносим за скобку ] = = (a-c)*(m - b + d), ч.т.д.
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ab + cd = m*(a-c),ab + cd = am - cm,
am - cm - ab - cd = 0, добавим к обеим частям (ad + bc),
ad + bc = (am - cm) - ab -cd + ad + bc = m*(a-c) + (bc - ab) + (ad - cd) = m*(a-c) - b*(a-c) + d*(a-c) = [ (a-c) выносим за скобку ] =
= (a-c)*(m - b + d), ч.т.д.