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

Answer 2

a + a x 2 + a x 3 + a x 4 +.....+ a x 49 + a x 50= 12750

a x 1 + a x 2 + a x 3 + a x 4 +........+ a x 49 + a x 50 =12750

a x ( 1 + 2 + 3 + 4 + ......+ 49 + 50 ) = 12750

a x ( 50 + 1 ) x 50 :2 =12750

a x 1275 = 12750

a = 12750 : 1275

a =     10

duyệt nha