(1)
sn=na1+n(n-1)d/2
=10×6+10×9×3 /2
=60+135
=195
(2)
sn=n(a1+an)/2
=8×(2+16)/2
=72
(3)
a10=a4+6d得
-2=10+6d
d=-2
a1=a4-3d=10-3×(-2)=16
a12=a10+2d=-2+2×(-2)=-6
sn=n(a1+an)/2
=12×(16-6)/2
=60
(1)
sn=na1+n(n-1)d/2
=10×6+10×9×3 /2
=60+135
=195
(2)
sn=n(a1+an)/2
=8×(2+16)/2
=72
(3)
a10=a4+6d得
-2=10+6d
d=-2
a1=a4-3d=10-3×(-2)=16
a12=a10+2d=-2+2×(-2)=-6
sn=n(a1+an)/2
=12×(16-6)/2
=60