Find the sum of the following: `1/7+9/7; 1/7+13/(-11)`

Without adding, find the sum of the following numbers : 1+3+5+7+9+11+13+15+17+19+21+23+25

Without adding, find the sum of the following numbers 1 + 3 + 5 + 7 + 9 + 11 + 13 + 15 + 17 =

Find the sum of n terms of the following series : 1+3+7+13 +......

Find the mode of the following numbers : 7,9,11,7,6,5,9,13

Compare the following numbers: 7/9'11/13

Find the sum to n terms of each of the following series : 1.3 .5 + 3.5.7 + 5.7.9+…

Determine the sum of following series upto n terms. 1.3+5.7+9.11+ …….

Find the sum of 1+3+5+7+…….+11

Find the sum to n terms of the series (1)/(1*3*5*7*9)+(1)/(3*5*7*9*11)+(1)/(5*7*9*11*13)+"......" . Also, find the sum to infinty terms.