The first number is 3, the second number is 6, the third number is 10, and the fourth number is 15. According to the law, what is the nth number?
Suppose the first number is a 1, the second number is a2, and so on.
a2=a 1+3
a3=a2+4
a4=a3+5
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an=a(n- 1)+n+ 1
Add both sides at the same time
sn-a 1 = s(n- 1)+(3+n+ 1)(n- 1)/2
an=3+(n- 1)(n+4)/2