Question

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Answer 1

•Here is your answer,

D= Term2 - Term1

d= -1 , a= 10

Sn= 155

So,

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

310= n( 20+ (n-1)-1)

310= 20n- n²+ 1n

0= n²-21n+310

0= n²+10n-31n+310

0=n(n+10) -31(n+10)

So, n= -10 or n= 31.

Therefore, n= 31, is your answer.