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

Correct answer:

x =  423
a7 =  69

Step-by-step explanation:

a=3 d=11 n=9 b=a+(n1) d=3+(91) 11=91 x=n (a+b)/2=9 (3+91)/2=423
a7=a+(71) d=3+(71) 11=69

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