Question 80963
Mary's annual salaries form an aritnmetic sequence. She earned $17,400 during her third year and $21,000 during her sixth year. 
a. Find her starting salary.
Since it is an aritmetic sequence the nth term = 1st term + (n-1)d
a(n)=a(1)+(n-2)d
The 3rd term is a(3)=a(1)+2d=17,400
The 6th term is a(6)=a(1)+5d=21000
You need to find a(1):
Subtract the 3rd term form from the 6th term form to get:
3d=3600
d=1200
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Substitute in the form a(1)+2d=17400 to find a(1)
a(1)+2(1200)=17400
a(1) = $15,000 (This is Mary's salary the 1st year.)
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b. Find her annual salary increases. 
"d" tells you the annual salary increase.
d= $1,200
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Cheers,
Stan H.