Sum of series with alternate signed squares of AP | function to calculate series sum ; Driver code
< ? php function seiresSum ( $ n , $ a ) { return $ n * ( $ a [ 0 ] * $ a [ 0 ] - $ a [ 2 * $ n - 1 ] * $ a [ 2 * $ n - 1 ] ) / ( 2 * $ n - 1 ) ; } $ n = 2 ; $ a = array ( 1 , 2 , 3 , 4 ) ; echo seiresSum ( $ n , $ a ) ; ? >