Find a1 of arithmetic sequence from given information.

a7 = -45 a15 = -77



Use the formula:



an = a1 + (n-1)d



 a7 = a1 + (7-1)d



-45 = a1 + 6d



 a15 = a1 + (15-1)d

 -77 = a1 + 14d



So you have this system of equations:



 -45 = a1 +  6d

 -77 = a1 + 14d



Can you solve that system of equations?

If not post again.



a1 = -21, d = -4



Edwin

AnlytcPhil@aol.com  

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