Question

1+3+...+37=...,please

Answer 1

A = 1
b = 2
un = 37
n = 19
sn = n/2 (a+un)
     = 19/2 (1+37)
     = 19/2 (38)
     = 19 ( 19)
     = 361

Answer 2

Un = a+(n-1)b
37 = 1+(n-1)2
37 = 1+2n-2
37 = 2n-1
37+1 = 2n
38/2 = n
19 = n

sn = n/2 (2a+(n-1)b)
sn = 19/2(2.1+(19-1)2)
sn = 19/2(2+36)
sn = 19/2.38
sn = 19.19
sn = 361