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245=155+(n-1)5
245=155+5n-5
245=150+5n
245-150=5n
95 =5n
19 =n
Sn=n/2(2.a+(n-1)b)
S19=19/2(2.155+(19-1)5)
=9,5(310+(18)5)
=9,5(310+90)
=9,5(400) = 3800 (a)