Common difference

The 4th term of an arithmetic progression is 6. If the sum of the 8th and 9th term is -72, find the common difference.

Correct result:

d =  -9.3333


a4=6 a8+a9=72 (a4+4d)+(a4+5d)=72  2a4+9d=72  d=(722 a4)/9=(722 6)/9=2839.3333=9.3333   Verifying Solution:  a1=a43 d=63 (9.3333)=34 k8=a1+7 d=34+7 (9.3333)=94331.3333 k9=a1+8 d=34+8 (9.3333)=122340.6667 s89=k8+k9=(31.3333)+(40.6667)=72

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