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 6th pentagonal number is $\qquad$.
【Key Point】Find the pattern 【Difficulty】Scoring
【Answer】 51
【Analysis】From triangle to hexagon, the first number is always 1, the second number increases by +1, the third number increases by +3 (1+2), the fourth number increases by +6 (1+2+3), the fifth number increases by +10 (1+2+3+4), the sixth number increases by +15 (1+2+3+4+5), the sixth triangular number is 21, so the sixth pentagonal number is $21+15+15=51$