Question

find the sum of first 15 terms whose n term is 9-5n​

Answer 1

Last term = 9-5n

a+ (n-1)d = 9-5n

a+ nd - d = 9-5 n

a - d = 9 and nd = -5n⇔ d = -5

a - (-5) = 9 ⇔ a = 4

l = 9-5(15) = 9-75 = -66

sum = N/2 (a+ l) = 15/2 (4-66) = -31*15 = -465

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Answer 2

the sum of first 15 terms whose n term is 9-5n is (-465)

Step-by-step explanation:

an = 9-5n

a1 = 9-5 = 4

a2 = 9-10 = -1

d = -5

so ,

S15 = 15/2 [2a +(15-1)d]

= 15/2 [ 2(4) + 14(-5)]

= 15/2 [ 8-70]

= 15/2 ×(-62)

= 15×(-31)

=(-465)