Question

What is the sum of the first 11 terms of the arithmetic series in which a1= -12 and d=5?

Answer 1

Hello :  
the terme general is : an = a1 +(n-1)d
 a11 = a1+(11-1)×5
a11 = -12+50
a11 = 38
the sum of the first 11 terms is : S11 = (11/2)(a1+a11)
S11 = (11/2)(-12+38)
S11 = 143