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a= u1 = 2. u5 = 10.
u5-u1 = 4b
10-2 = 4b
8=4b. b= 2
un = bn + (a-b)
u20 = 2x20 + (2-2)
u20 = 40
s20= 20/2 (2+40)
s20 = 420