Question

The A.P. in which 4th term is -15 and 9th term is -30.Find the sum of the first 10 numbers.

Answer 1

Answer:

-195

Step-by-step explanation:

A4=a+3d =-15

A9= a+8d= -30

Subtracting both eqs.

-5d=15

d= -3

Put the value of d in first eq

a-9=-15

a=-6

S10=10/2(2*(-6)+9(-3))

=5(-12-27)

=5(-39)

=-195

Answer 2

Answer:

a+3d= -15 --------->eq 1

a+8d= -30 --------->eq 2

Now on subtracting both eq

a+8d-a-3d= -30+15

5d= -15

d= -3

Now putting d value in eq 1

a-9= -15

a= -6

Sum= 10/2[2×(-6)+9×(-3)]

=5[-12-27]

=5×(-39)= -195