Hello Vidisha , we know sum of squares of first n natural number = n ( n +1 )( 2n +1 )/6 1² + 2² + 3² + ... + 2005² = 2005 × 2006 × 4011...

Hello Kinshuk ! PQR is an Isosceles triangle so PE bisects QR QE = ER In ∆PQE => QE² = PQ² - PE² = 7² - PE² In ∆PDE , DE² = PD² - PE² = 5² - PE²...

Alternate Approach :

Hello Abhi !