Question

without actual adding find the sum of (a) 1+3+5+7+9+11+13. (b) 15+17+19+21+23+25​

Answer 1

Answer:

(a) 49 (b) 120

Step-by-step explanation:

Sum of first n odd natural numbers sum is n^2

therefore, sum of first 7 odd natural numbers is 49.

From  there 1+3+5+7+9+11+13+15+17+19+21+23+25 are 13 numbers

sum of first 13 odd numbers is 169.

But to find the sum of 15+17+19+21+23+25​ subtract the sum of first 7 odd natural numbers from the sum of first 13 odd natural numbers.

Therefore, 15+17+19+21+23+25​ = 169-49 = 120