Question

please solve this quickly....

Answer 1

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S10 = -60 
S15 = -165 

S10 = n/2 ( a1 + (n-1)d) 
-60  = 5 ( a1 + 9d ) 
-60  = 5a1 + 45d 
DBS by 5  
-12  = a1  + 9d ----(1) 


Similarly , 

-165 = 15/2 (a1 + 14d ) 
-165 = (15a1 + 210 d )/ 2 
-330 =  15a1 + 210d 
-66   = 3a1    + 42d -----(2) 


MBS by 3 in equation 1 

we got 

-36 = 3a1 + 27d ----(3) 

Subtracting equation (3) from (2) 

We got 

-30 = 15d 

-2  = d 

put value of d in equation 1 we got 

a1 = 6 

Sum of n terms 
  = a1 + (n-1)d 
  = 6 + (n-1)-2