a+a*2+a*3+....+a*50=12750
=>a*(1+2+3+....+50)=12750
Tổng trong ngoặc=[50x(50+1)]:2=1275
=>a=12750:1275=10
vậy a=10
a + a x 2 + a x 3 + a x 4 + ..... + a x 49 + a x 50 = 12750
a x ( 1 + 2 + 3 + .... + 50 ) = 12750
a x 1275 = 12750
a = 12750 : 1275
a = 10
\(a+a\times2+a\times3+a\times4+...+a\times49+a\times50=12750\)
\(a\times\left(1+2+3+...+50\right)=12750\)
\(a\times1257=12750\)
\(a=12750\div1275\)
\(a=10\)
