Subtraction equals 25= last term +(n- 1)*d/2.
The first item = the last item -(n- 1)*d
Substitute a formula to eliminate the first term =[2 last term -(n- 1)*d]*n/2= last term *n-n(n- 1)*d/2=325.
The last term is eliminated in two forms: 25-(n- 1) * d/2, that is, 25n-(n-1) n * d/2-n (n-1) * d/2 = 325.
Solve n= 13, and further solve d=5.
1. How many courses are there in Grade One?
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