Question

find sum of first 17 terms of an AP whose 4th and 9th terms are - 15 and -30 respectively​

Answer 1

Answer:

Hey friend, your answer is -

Here, t4 = -15 and t9 = -13

tn = a + (n -1 ) d.........formula

t4 = a + 3d

a + 3d = -15......... (1)

Similarly, a + 8d = -13.......... (2)

Subtract equation 1 from 2 ,

a + 8d = -13

-

a + 3d = -15

------------------

5d = 2

d = 2 /5

Substitute d= 0.4 in equation 1 ,

a + 3×2/5 = -15

5a + 6 = -75

5a = -81

a = -81 / 5

Sn = n/2 (2a + (n-1)d)

S17 = 17 / 2 (2× -81 /5 +(17-1)×2/5)

= 17/2 (-162 /5 + 32 / 5

= 17 / 2 × -130 / 5

= - 442 / 2

= - 221

S17 = - 221

⚡⚡Hope it will help you.⚡⚡

Answer 2

Answer:

Hope this helps you ✌️✌️