В2
1.
а1-? d-?
a7+a3=-8
a1+(7-1)d+a1+(3-1)d=-8
2a1+8d=-8
a1+4d=-4
a8-a5=-6
a1+(8-1)d-(a1+(5-1)d)=-6
a1+7d-a1-4d=-6
3d=-6
d=2
a1+4*2=-4
a1=-12
2.
S14-?
an=6n-4
a1=6*1-4=2
a14=6*14-4=80
s14=(2+80)*14/2=82*7=574
3.
a3+a6=a4+a5=2.2+(-1.8)=0.4
B4
1.
a6-a4=-5
a1+(6-1)d-(a1+(4-1)d)=-5
a1+5d-a1-3d=-5
2d=-5
d=-2.5
a10+a2=-46
a1+(10-1)d+a1+(2-1)d=-46
2a1+10d=-46
a1+5d=-23
a1+5*(-2.5)=-23
a1=-23+12.5
a1=-10.5
2.
S24-?
an=3/4n- 5/8=0.75n-0.625
a1=0.75*1-0.625=0.125
a24=0.75*24-0.625=17,375
S24=(0.125+17.375)*24/2=17.5*12=35*6=70*3=210
3.
a7+a23=a14+a16=-7+(-1)=-8
a15=(a14+a16)/2=-8/2=-4