A11=6 a20=12 a1=? d=?

Question

A11=6 a20=12 a1=? d=?

Answer 1

A11=a1+10d=6
a20=a1+19d==12

a1+19d=a1+10d+9d=12

6+9d=12
9d=6
d=6:9
d=2/3
a1+19*2/3=12
a1+12 2/3=12
a1=12-12 2/3
a1=-2/3

Comments

12-6=6, а не 3

Answer 2

A11=a1+10d
a20= a1+19d

a1+10d=6
a1+19d=12

6-10d=12-19d
-10d+19d=12-6
9d=6
d=2/3
a1=6-10d= 6-20/3= 18/3-20/3= -2/3
Ответ: a1= -2/3; d= 2/3