Giải câu 2 trang 46 sách toán VNEN lớp 7 tập 2.
M + N = (3xyz – 3x2 + 5xy – 1) + (5x2 + xyz – 5xy + 3 – y)
= 3xyz – 3x2 + 5xy – 1 + 5x2 + xyz – 5xy + 3 – y
= (3xyz + xyz) + (– 3x2+ 5x2) + (5xy – 5xy) + (-1 +3) –y
= 4xyz + 2x2 – y + 2
M – N = (3xyz – 3x2 + 5xy – 1) – (5x2 + xyz – 5xy + 3 – y)
= 3xyz – 3x2 + 5xy – 1 - 5x2 - xyz + 5xy - 3 + y
= (3xyz- xyz) – (3x2 + 5x2) + (5xy + 5xy) – (1+3) + y
= 2xyz – 8x2 + 10xy + y – 4
N – M = (5x2 + xyz – 5xy + 3 – y ) – (3xyz – 3x2 + 5xy – 1)
= 5x2 + xyz – 5xy + 3 – y - 3xyz + 3x2 - 5xy + 1
= (5x2+ 3x2 ) + (xyz- 3xyz) – (5xy + 5xy) – y +( 3+ 1)
= 8x2 – 2xyz – 10xy – y + 4