Answer:
[tex]a \: b \: d \: f \\ mathematical \: sentence \: is \: one \: \\ which \: has \: a \: sign \: like = < \: \: \: \: \: \\ > \: \: \: \: \: \leqslant \: \: \: \: \: \geqslant [/tex]
it must always have one of the sign
