If A=-7 ‚ D=4‚tn=73 find n and sn
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n = 21 :Sn = 693
Step-by-step explanation:
An=a+(n-1)d
73= -7 + ( n-1 ) 4
73+7=4n-4
80 +4 =4n
84/4= n
n = 21.
Sn= n/2 [2a + (n-1) d]
Sn = 21/2 [2(-7) + (21-1) 4]
Sn = 21/2 [-14+20(4)]
Sn = 21/2 [-14+80]
Sn = 21/2 (66)
Sn = 21(33)
Sn = 693
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