a1=6,a13=42, c10=?
a13=a1+12d, 12d=a13-a1, 12d=42-6, 12d=36, d=3
a10=a1+9d, a10=6+27,a10=33
c10=10(a1+a10)/2, c10=5(6+33)=5.39=195, c10=195
c10=195
Объяснение:
cn=n(a1+an)/2
