The third term of an A.P is 10 and the sixth term is 43 find: the sum of the first 20 terms with shown working

Answer 1

The common difference of the A.P is given by

d = (43 - 10)/(6 - 3) = 33/3 = 11

The nth term of the A.P is given by

a_n = a_1 + (n - 1)d

The first term of the A.P is given by

a_1 = 10 - (3 - 1)11 = -2

The sum of the first 20 terms is given by

S_20 = (20/2)[2a_1 + (20 - 1)d]

S_20 = (20/2)[2(-2) + (20 - 1)11]

S_20 = (20/2)[-40 + 209]

S_20 = (20/2)[169]

S_20 = 840

Answer 2

What was the question?