{a1+a5=20
{a1+a3=17
a5=a1+4d
a3=a1+2d
{a1+a1+4d=20
{a1+a1+2d=17
{2a1+4d=20
{2a1+2d=17
a1+2d=10
a1=10-2d
2(10-2d)+2d=17
20-4d+2d=17
20-2d=17
-2d=-3
2d=3
d=1.5
a1=10-2*1.5=7;
ответ: a1=7.
{a1+a5=20
{a1+a3=17
a5=a1+4d
a3=a1+2d
{a1+a1+4d=20
{a1+a1+2d=17
{2a1+4d=20
{2a1+2d=17
a1+2d=10
a1=10-2d
2(10-2d)+2d=17
20-4d+2d=17
20-2d=17
-2d=-3
2d=3
d=1.5
a1=10-2*1.5=7;
ответ: a1=7.