S3 = (2a1+2d)/2 * 3 = (a1+d)3=9S6 = (2a1 + 5d)/2 * 6 = (2a1+5d)3=-63(a1+d)3=9(2a1+5d)3=-63a1+d=32a1+5d=-212a1+2d=62a1+5d=-212a1+5d-2a1-2d=-21-63d=-27d=-9a1=3-d=3+9=12S10=(2a1+9d)/2 * 10 = (2a1+9d)5 = (24-81)5=-57*5=-285
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S3 = (2a1+2d)/2 * 3 = (a1+d)3=9
S6 = (2a1 + 5d)/2 * 6 = (2a1+5d)3=-63
(a1+d)3=9
(2a1+5d)3=-63
a1+d=3
2a1+5d=-21
2a1+2d=6
2a1+5d=-21
2a1+5d-2a1-2d=-21-6
3d=-27
d=-9
a1=3-d=3+9=12
S10=(2a1+9d)/2 * 10 = (2a1+9d)5 = (24-81)5=-57*5=-285