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Which is an equation for the nth terms of the sequence 12,15,18,21

Determine the number of girls in a math class with 87 boys if there are 25 girls for every 75 boys

36x2 + 49y2 = 1,764 the foci are located at: a) (-√13, 0) and (√13,0) b) (0, -√13) and (0,√13) c) (-1, 0) and (1, 0)