Question

The 10th and 15th term of an Ap are -5 and -7½ respectively, what is the sum of the first 20 term

Answer 1

shoe full work

Answer 2

You must learn the main definitions of sequences, since you will need them

10th term is -5 ----> a + 9d = -5
15th term is -7.5 ---> a + 14d = -7.5
subtract them:
5d = -2.5
d = -.5
then in a + 14d = -7.5
a - 7 = -7.5
a = -.5

now you have a and d, using sum(n) = (n/2)(2a + (n-1)d )
sum(20) = (20/2)(2(-.5) + 19(-.5) )
= ....

let me know what you get

Answer 3

40

Answer 4

-105