Question

初中数学:先化简,再求值

1. 18a²-{-6a²+[5a-4a²-(a²-2a²)]+15a²-5a}，其中a=5分之1
2.已知M=2x²+3kx-2x+6,N=-x²+kx+2,且3M+6N的值与X无关,求k的值

Answer 1

1. 18a²-{-6a²+[5a-4a²-(a²-2a²)]+15a²-5a}
=18a²-{-6a²+[5a-4a²-a²+15a²-5a}
=18a²-{-6a²+10a²}
=14a²
当a=1/5时，原式=14×1/25=14/25
2. 3M+6N
=3（2x²+3kx-2x+6）+6（-x²+kx+2）
=6x²+9kx-6x+18-6x²+6kx+12
=(15k-6)x+30
∵3M+6N的值与X无关
∴15k-6=0
k=2/5

Answer 2

1. 18a²-{-6a²+[5a-4a²-(a²-2a²)]+15a²-5a}
=18a²+6a²-5a+4a²+a²-2a²-15a²+5a
=11a²
=11/25
2. 3M+6N=3(2x²+3kx-2x+6)+6(-x²+kx+2)
=6x²+9kx-6x+18-6x²+6kx+12
=(15k-6)x+30
因为3M+6N的值与X无关,所以
15k-6=0
则k=6/15=2/5