Ratio and Proportion

1. if  a:b=c:d then we write a:b::c:d and say a,b,c,d are in proportion.

    Here  ’a’  and  ‘d’ are called extrems while ‘b’ and ‘c’ are called means.

     2. Product of means(bxc)=product of extreams(axd)

     3.  If  a:b=c:d then ‘d’ is called fourth proportional .

     4.  If a:b=b:c then ‘c’ is called third proportional

     5.  (ab)^(1/2) is mean proportional between a and b.

     6.   If a:b=c:d

then componendo rule:- (a+b)/b=(c+d)/d

   dividendo rule :-              a/(a-b)=c/(c-d)

componendo and dividend rule:- (a+b)/(a-b)=(c+d)/(c-d)