f(x+1)−f(x)=3x
x=1→f(2)−f(1)=3.1
x=2→f(3)−f(2)=3.2
x=3→f(4)−f(3)=3.3
⋮
x=20→f(21)−f(20)=3.20 taraf tarafa toplanırsa
f(21)−f(1)=3.(1+2+3+4+...+20)=630⇒f(21)=632 .
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