✔ 最佳答案
1st term= 10
2nd term=17= (2x1+5)+10
3rd term=26 = (2x2+5)+17= (2x2+5)+(2x1+5)+10
4th term=(2x3+5)+(2x2+5)+(2x1+5)+10
so,
7th term
=(2x6+5)+(2x5+5)+...+(2x1+5)+10
=(2x6)+(2x5)+...+(2x1)+ 5x6 +10
=2(6+5+4+3+2+1) +40
=2(21)+40
=42+40
=82
As 1+2+3+...+n =[(1+n)n]/2