9. The ancient Greek mathematicians associated natural numbers with polygons in the following way,
Triangular numbers: $1,3,6,10,15$, $\qquad$
Square numbers: $1,4,9,16,25$, $\qquad$
Pentagonal numbers: 1, 5, 12, 22, 35, $\qquad$
Hexagonal numbers: $1,6,15,28,45$, $\qquad$
According to the above sequence, the 8th triangular number is $\qquad$ .
【考点】Find the Pattern
【难度】 $\hat{\imath} \hat{\imath} \hat{\imath}$

【答案】 36
【解答】Triangle: $1 、 1+2 、 1+2+3 、 1+2+3+4 、 1+2+3+4+5 、 1+2+3+4+5+6 、 \cdots \cdots 、 1+2+3+\cdots+8=36$.