Question

(3x2y加5xy)减(x2y减2(xy减x2y))其中x等于负1,y等于7分之1？

Answer 1

解:(3x^2y+5xy)一[x^2y一2(xy一x^2y)]
=3ⅹ^2y+5xy一[x^2y一2ⅹy+2x^2y]
=3x^2y+5xy一x^2y+2xy一2x^2y
=(3一1一2)x^2y+(5+2)xy
=7ⅹy，
当ⅹ=一1，y=1/7时，
原式的值
=7x(一1)x(1/7)
=一1。