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Verified answer
A) a=6b=5
Un=101
Un=a+(n-1)b
101 = 6+(n-1).5
101 = 6 + 5n - 5
101 = 5n + 1
100 = 5n
n = 20
==>
Sn = n×(a+Un) : 2
S20 = 20 × (6+101) :2
S20 = 20 × 107 : 2
S20 = 1.070
b) a=5
b=3
Un=77
Un=a+(n-1)b
77 = 5+(n-1).3
77 = 5 + 3n - 3
77 = 3n + 2
75 = 3n
n = 25
==>
Sn = n×(a+Un) : 2
S25 = 25 × (5 + 77) : 2
S25 = 25 × 82 : 2
S25 = 1.025
c) a=7
b=4
Un=103
Un=a+(n-1)b
103 = 7+(n-1).4
103 = 7 + 4n - 4
103 = 4n + 3
100 = 4n
n = 25
==>
Sn = n×(a+Un) : 2
S25 = 25 × (7+103) :2
S25 = 25 × 110 : 2
S25 = 1.375
d) a=4
b=6
Un=58
Un=a+(n-1)b
58 = 4+(n-1)6
58 = 4 + 6n - 6
58 = 6n - 2
54 = 6n
n = 9
==>
Sn = n×(a+Un) : 2
S9 = 9 × (4+58) : 2
S9 = 9 × 62 : 2
S9 = 279
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