Question

the ap in which 4 th term is -15 and 9 th term is -30 find the sum of first 10 numbers​

Answer 1

a4 = -15

a9 = -30

then,

a + 3d = -15 equation 1

a + 8d = -30 equation 2

solve the equation,

-5 d = 15

d = 15/-5 = -3

putting the value of d in eq 1

so,

a + 4d = -15

a + 4*-3 = -15

a = -15+12 = -3

so,

a10 = a + 9d = -3 + 9*-3

= -3 - 27 = -30