a10 = 5, a11 = -4, d-?, a5-?, a21-?

а10=а1+9dа1=а10-9d

а11=а1+10аa1=a11-10d

a10-9d=a11-10d

5-9d=-4-10d

10d-9d=-4-5

d= -9

a1= 5 - 9×(-9)=5+45=50

a5=a1+4d= 50 + 4 ×(-9)= 50-36=14

a21=a1+20d=50+20×(-9)=50-180=-130