Question

Tìm a biết :a+a*2+a*3+a*4+ ... +a*49+a*50=12750

Answer 1

=> a . ( 1 + 2 + 3 + .... + 49 + 50 ) = 12750

=> a . { [ 50.( 50 + 1 ) ] : 2 } = 12750

=> a. ( 2550 : 2 ) = 12750

=> a. 1275 = 12750

=> a = 12750 : 1275

=> a = 10