IPA: /ˌsʌplɪˈmɛn.tər.i/
KK: /sʌplɪˈmɛntəri/
Something that is added to provide extra support or information.
The teacher provided supplementary materials to help students understand the topic better.
Supplementary → It is formed from "supplement" (from Latin "supplementum", meaning something added) and the suffix "-ary" (meaning pertaining to or connected with). The word "supplementary" refers to something that is added to enhance or complete something else.
Think of something that is 'added' ('supplement') to make it better or more complete, and the '-ary' means it relates to that addition.