Question

the sum of the arithmetic sequence 51+52+53......+70​

Answer 1

Answer:

the sum of the arithmetic sequence is 1210

Answer 2

Answer:

a1 = 51

a2 = 52

an = 70

d = a2-a1

  = 52-51

    = 1

an = a+(n-1)d

 70  = 51 +(n-1)1

70-51 = n-1

19 = n-1

n = 19+1

n=20

Sn = n/2[2a+(n-1)d]

S20 = 20/2[2×51 +(20-1)1]

       = 10 [102 +19]

      = 10×121

      = 1210