Use the appropriate formula to find the indicated figurate number.
-the 15th octagonal number
A) $\mathrm { O } _ { 15 } = 225$
B) $\mathrm { O } _ { 15 } = 82.5$
C) $\mathrm { O } _ { 15 } = 645$
D) $\mathrm { O } _ { 15 } = 187.5$

Question

Quizplus · Accepted Answer

The formula for the nth octagonal number is $O_n = n(3n - 2)$. Substituting 15 for n gives $O_{15} = 15(3*15 - 2) = 15(45 - 2) = 15*43 = 645$.

Use the Appropriate Formula to Find the Indicated Figurate Number O15=225\mathrm { O } _ { 15 } = 225O15​=225

Use the Appropriate Formula to Find the Indicated Figurate Number $\mathrm { O } _ { 15 } = 225$