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

a x 1275 = 12750

a = 10

=> 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

a*(1+2+3+.......................+50)=12750

a*1275=12750

a=12750:1275

a=10

a + a × 2 + a × 3 + a × 4 + … + a × 49 + a × 50 = 12750

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

a x 1275 = 12750

a              = 12750 : 1275

a              = 10

a + a × 2 + a × 3 + a × 4 + … + a × 49 + a × 50 = 12750

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

a x 1275 = 12750

a              = 12750 : 1275

a              = 10

=> 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

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          

a + 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  1275                                        =  12750

   a =  12750 : 1275

   a = 10

Vậy a = 10

Giải:

Ta có:a*1+a*2..................a*50=12750

Vậy a*(1+2+3+..................+50)=12750 

        a=12750:(1+2+3+..................+50)

        a=12750:1275

        a=10