1, odd terms are positive and even terms are negative.

2. What is the number of molecules?

3. The denominator is the product of two adjacent odd numbers, 2n- 1 and 2n+ 1 respectively, that is, the first denominator is 1× 3 and the second denominator is 1×3.

3×5, and the third denominator is 5×7. . . . . . And so on.

1/3, -2/ 15, 3/35, -4/63 The following are 5/99, -6/ 143.