Chọn câu đúng
A. x ( x + 1 ) 4 + x ( x + 1 ) 3 + x ( x + 1 ) 2 + ( x + 1 ) 2 = ( x + 1 ) 5
B. x ( x + 1 ) 4 + x ( x + 1 ) 3 + x ( x + 1 ) 2 + ( x + 1 ) 2 = ( x + 1 ) 6
C. x ( x + 1 ) 4 + x ( x + 1 ) 3 + x ( x + 1 ) 2 + ( x + 1 ) 2 = ( x + 1 ) 4 x - 1
D. x ( x + 1 ) 4 + x ( x + 1 ) 3 + x ( x + 1 ) 2 + ( x + 1 ) 2 = ( x + 1 ) 4 x + 2
Ta có
x ( x + 1 ) 4 + x ( x + 1 ) 3 + x ( x + 1 ) 2 + ( x + 1 ) 2 = x ( x + 1 ) 4 + x ( x + 1 ) 3 + ( x + 1 ) 2 ( x + 1 ) = x ( x + 1 ) 4 + x ( x + 1 ) 3 + ( x + 1 ) 3 = x ( x + 1 ) 4 + ( x + 1 ) 3 ( x + 1 ) = x ( x + 1 ) 4 + ( x + 1 ) 4 = ( x + 1 ) 5
Đáp án cần chọn là: A