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 + ... + 50 ) = 12750
a x 1275 = 12750
a = 12750 : 1275
a = 10
a+ax2+ax3+ax4+ax5+...+ax49+ax50=12750
ax1+ax2+ax3+ax4+...+ax49+ax50=12750
ax(1+2+3+4+...+49+50)=12750
ax1275=12750
a =12750:1275
a =10
a+1a+2a+3a+...+49a+50a=12750
a(1+2+3+...+49+50)=12750
a[25(50+1)]=12750
1275a=12750
a=12750/1275
a=10
Vậy số cần tìm a=10
