Find1 for me following arithmetic sequence S20740 a20 75 The

Find_1 for me following arithmetic sequence. S_20-740, a_20= 75 The form of the formula for the sum of the first n terms of an arithmetic sequence that would n work best here is S_n = n/2 (a_1 + a_n). Use the given values for S_20 and a_20 and the formula S_n = n/2(a_1 + a_n) with n equal to 20 to

Solution

S20 = 740 ; a20 = 75

S20 = (an +a1)*n/2

740 = ( a20 +a1)*20/2

74 = a20 +a1

a1 = 74 -a20 = 74 - 75

a1 = -1