If the sum of first 10 terms of and ap is 140 and the sum of first 16 terms is 320, then find the sum of first m terms.
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Step-by-step explanation:
S10 = 140 ; S16= 320
S10= 10/2 [2a+9d]
140= 10/2 [2a+9d]
28 = 2a+ 9d (1)
S16 = 16/2 [2a+15d]
320 = 16/2 [2a+15d]
40 = 2a+15d (2)
Solving both equations we get :
d=12 ; a= -40
Sn =n/2 [2a+(n-1)d]
By putting values of a and d in above equation you will get answer.
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