\(a+a.2+a.3+...+a.49+a.50=12750\)

\(\Leftrightarrow a\left(1+2+3+...+49+50\right)\)

\(\Leftrightarrow1275a=12750\)\

\(\Leftrightarrow a=\frac{12750}{1275}=10\) 

     Vậy \(a=10\)

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a = 10

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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