Question

The nth term of an A.P. is 6-n, the sum of first 15 terms is
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Answer 1

Answer:

-45

Step-by-step explanation:

Let n be 1

so 6-n,

=> now a = 5 , d= -1 , n = 15

=> a15= a+(n-1)*d

=> a15 = 5+(15-1)*-1

=> a15 =  5+(-14)

=> a15 = 5-14

Therefore 15th term is -11

now we can use S=n/2*[a+l]

S=n/2*[a+l]

S= 15/2*[5+(-11)]

S= 15/2*[5-11]

S= 15/2*[-6]

S= 15*[-3]

Therefore S= -45