S3=9;S6=-63 найти:S10?

Question

S3=9;S6=-63 найти:S10?

Answer 1

2a1+2d
S3 = ----------- • 3 = (a1+d)•3
2

2a1+5d
S6 = ----------- • 6 = (a1+2,5d)•6
2

{3a1+3d=9 |•(-5)
{6a1+15d=–63

{-15a1–15d=–45
{6a1+15d=–63

–9a1=–108
a1=12
3•12+3d=9
3d=–27
d=–9
2a1+9d
S10 = ------------ • 10 = (a1+4,5d)•10 =
2

= (12+4,5•(-9))•10 = –285

Comments

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Answer 2

S7=441
S8= -3087
S9= 21609
S10= -151263