Use the Appropriate Formula to Find the Indicated Figurate Number $\mathrm { T } _ { 6 } = 18$

Multiple Choice

Use the appropriate formula to find the indicated figurate number.
-the 6th triangular number

A) $\mathrm { T } _ { 6 } = 18$
B) $\mathrm { T } _ { 6 } = 15$
C) $\mathrm { T } _ { 6 } = 42$
D) $\mathrm { T } _ { 6 } = 21$