Q.26) If an-3-4n//show that al, a2, аз,form an APAlso find S

1.	Q.26) If an-3-4n//show that al, a2, аз,form an APAlso find S
Answer» If tn is the nthterm...then... let n = 1, then 1stterm, t1(or 'a') = 3 - 4 = -1 now, let n = 2, then 2ndterm, t2= 3 - 8 = -5 therefore, common difference, d = t2- t1= -5 - (-1) = -4 now, let n = 20, then, last term, t20= 3 - 80 = -77 so, sum of first 20 terms, S20= n/2 [2a + (n-1)d]= n/2 [a + a + (n-1)d] = n/2 [a + l] = 20/2 [(-1) + (-77)] = 10 (-78) = -780 plz full image send

Answer»

If tn is the nthterm...then...

let n = 1, then 1stterm, t1(or 'a') = 3 - 4 = -1

now, let n = 2, then 2ndterm, t2= 3 - 8 = -5

therefore, common difference, d = t2- t1= -5 - (-1) = -4

now, let n = 20, then, last term, t20= 3 - 80 = -77

so, sum of first 20 terms, S20= n/2 [2a + (n-1)d]= n/2 [a + a + (n-1)d]

= n/2 [a + l]

= 20/2 [(-1) + (-77)]

= 10 (-78)

= -780

plz full image send

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