Cho a b c d, Cho a/b=c/d, cho a/b = c/d, Cho a / b = c /d, Cho a/ b = c/ d, Cho a + b + c + d, Cho a/b=c/d., cho a/b=c/d., Cho a/b= c/d, Cho a/b =c/d, Cho: a/b = c/d