2Gether Reunites on L.A. Sidewalk ... SINGS CALCULUS!!!!!

It was 10 years in the making ... 10 YEARS ... but the guys from 2gether finally performed their hit song "Calculus" yesterday ... on an L.A. sidewalk ... and TMZ has the footage!!!!

Jerry ... Chad ... Doug and Mickey were hangin' outside of the Newsroom Cafe ... talking about their upcoming reunion show ... when we asked for a sneak preview ... and the guys delivered!

TMZ broke the story ... the MTV boy band is all set to officially reunite on stage at a performance in L.A. next month ... where they'll hopefully perform their new song, "I Bet You Regret Having Sex with Me Now."