Question

Find the sum of 5square +6square+7square+...... +20square

Answer 1

Answer:

2815

Step-by-step explanation:

We know that summation of squares upto 'n' terms is n(n+1)(2n+1)/6.

Sn=5²+6²+7²+.....20²

Adding and subtracting 1²+2²+...+5²

Sn=1²+2²+3².....20²-(1²+2²+..+5²)

Sn=20(21)(41)/6-5(6)(11))/6

=2870-55

=2815

Answer 2

Answer:

answer is 2815

Step-by-step explanation:

Sn=5^2+6^2+7^2+.....20^2

adding and subtracting 1^2+2^2+3^2+....+5^2

Sn=1^2+2^2+3^2+...20^2-(1^2+2^2+.....+5^2)

Sn=20(21)(41)/6(6-5)(6)(11)/6

= 2870-55

= 2815