Giải:
1.2 + 2.3 + 3.4 + ... + 49.50 + $\frac{1}{2}$.$x$ = 210
Đặt: A = 1.2 + 2.3 + 3.4 + ... + 49.50
3A = 3 (1.2 + 2.3 + 3.4 + ... + 49.50
3A = 1.2.3 + 2.3.3 + 3.4.3 + ... + 49.50.3
3A = 1.2.(3-0) + 2.3.(4-1) + 3.4.(5-2) + ... + 49.50.(51-48)
3A = 1.2.3 - 0.1.2 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + ...+ 49.50.51 - 48.49.50
3A = 49.50.51
A = 49.50.51 : 3
A = 41650
Ta có: 41650 + $\frac{1}{2}$.$x$ = 210
$\frac{1}{2}$.$x$ = 210 - 41650
$\frac{1}{2}$.$x$ = -41440
$x$ = -41440 : $\frac{1}{2}$
$x$ = -82880
Vậy $x$ = -82880
#Yayoi~ #Chúc bạn học tốt nha!
