Question

if the sum of first 8 terms of ap is 136 & that of first 15 terms is 465 than find the sum of first 25 terms
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Answer 1

Answer:

465-136

=329

465+386

=851

Answer 2

Answer:

S 25 = n / 2 [ 2a + ( n- 1) d]

Step-by-step explanation:

S 15 = a + 14 d .....( i)

S 8 = a + 7 d ....... (ii)

now subtract i from ii

given that

d= 229 /7

put d in i equation

gives a= 7

now put a and d in formula

25/ 2 [ 2×7 + 24 × 229 /7 ]

= 960.14

I hope it's help you