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❓ Question Evaluate the sum of the series: S = 2 × 1 × ( 20 4 ) − 3 × 2 × ( 20 5 ) + 4 × 3 × ( 20 6 ) − 5 × 4 × ( 20 7 ) + ⋯ + 18 × 17 × ( 20 20 )…

❓ Question Let a n a_n a n ​ be the n th n^{\text{th}}  term of an A.P. If S n = a 1 + a 2 + ⋯ + a n = 700 ,    a 6 = 7 S_n=a_1+a_2+\cdots+a_n=700,…

❓ Question If the sum of the 2nd, 4th and 6th terms of a G.P. of positive terms is 21 and the sum of its 8th, 10th and 12th terms is 15309 , the…