First we find the common difference...to do this we subtract the first term from the second term. -7 - (-1) = -7 + 1 = -6
now we are going to find the 10th term an = a1 + (n-1)*d <== formula for finding any term in arithmetic series a1 = 1st term, d = common difference, n = term we want to find now we sub a10 = -1 + (10 -1) * -6 a10 = -1 + (9 * -6) a10 = -1 - 54 a10 = - 55
now we will find the sum Sn = (n (a1+ an)) / 2 <== formula for finding the sum S10 = (10(-1 - 55))/2 S10 = (10(-56) / 2 S10 = -560/2 S10 = - 280
