Which statement must be true? . 1.) if a parallelogram is not a rectangle, then it is not a square. 2.) if a parallelogram is not a square, then it is not a rectangle. 3.) all rectangles are squares. 4.) if a parallelogram is a rectangle, then it is a square.

1) False.parallelogram is not a rectangle but a rectangle is considered a special case of a parallelogram
2) False :Quadrilaterals are four-sided polygons. All rectangles are quadrilaterals.
3.) False.All rectangles are not squares.
4) True. A square is a parallelogram with four congruent sides and four right angles.

It is the first choice.

Step-by-step explanation:

This is a sequence where

the next term is obtained by adding 6.

first term = -9 + 6(0) = -9

second term = -9 + 6(1) = -3

third term = -9 + 6(2) = 3

4th term = -9 + 6(3)  = 9  

and 81 = -9 + (6*15).

So the answer is  Sigma (-9 + 6n)  from n= 0 to n = 15.