evaluate 5^2 + 6^2 + 7^2 + ....... + 20 ^ 2 .plzz Solve this
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1
Answer:
2840
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Step-by-step explanation:
Useful formula:
1² + 2² + 3² + ... + n² = n ( n + 1 ) ( 2n + 1 ) / 6
So
5² + 6² + ... + 20²
= ( 1² + 2² + ... + 20² ) - ( 1² + 2² + 3² + 4² )
= 20 × 21 × 41 / 6 - 4 × 5 × 9 / 6
= 10 × 7 × 41 - 2 × 5 × 3
= 2870 - 30
= 2840
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