Question

an=9-5n
find the sum of the 15terms​

Answer 1

Step-by-step explanation:

a1=9-5=4

a2=9-10=-1

d=a2-a1= -1-4= -5

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

n = 15

S15 = 15/2[4+(15-1)(-5)]

S15=15/2[4+(-70)]

S15=15/2(-64)

S15=15×(-32)

S15=(-480)

Answer 2

Given: an = 9 – 5n

Taking n = 1,

a1 = 9 – 5(1) = 9 – 5 = 4

Taking n = 2,

a2 = 9 – 5(2) = 9 – 10 = -1

Taking n = 3,

a3 = 9 – 5(3) = 9 – 15 = -6

Therefore the series is 4, -1, -6, …

So, a = 4, d = a2 – a1 = -1 – 4 = -5

Now, we have to find the sum of the first 15 terms of the AP ,so it is mentined step by step in the image attached above.

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