Students

In the front row sitting three students and in every other row 11 students more than the previous row. Determine how many students are in the room when the room is 9 lines, and determine how many students are in the seventh row.

Correct result:

x =  423
a₇ =  69

Solution:

$a=3 \ \\ d=11 \ \\ n=9 \ \\ b=a+(n-1) \cdot \ d=3+(9-1) \cdot \ 11=91 \ \\ x=n \cdot \ (a+b)/2=9 \cdot \ (3+91)/2=423$

$a_{7}=a+(7-1) \cdot \ d=3+(7-1) \cdot \ 11=69$

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