Решите уравнение х+2х+...+20х=2100

Question

Решите уравнение х+2х+...+20х=2100

Answer 1

X+2x+3x+4x+5x+6x+7x+8x+9x+10x+11x+12x+13x+14x+15x+16x+x17x+18x+19x+20x=2100;
210x=2100;
x=10

Answer 2

S (a1+aN)*N/2
х*(1+2+3...+20)= 2100
х*210=2100
х=2100:210
х=10