Question

(an) если a1=3 d=2.5

Answer 1

An=a1+(n-1)*d
a₂₂₇=3+(227-1)*2.5=3+226*2.5=3+565=568
Sn=(2a1+(n-1)*d)*n/2
S₂₁=(2*3+20*2.5)*21/2=(6+50)*21/2=56*21/2=28*21=588