C-11-Q140mediumsingle_mcqa−cb+c=a−cd+a\dfrac{a-c}{b+c} = \dfrac{a-c}{d+a}b+ca−c=d+aa−c থেকে পাই —aa=ca = ca=c অথবা a+b+c+d=0a+b+c+d = 0a+b+c+d=0