已知S+T=21,3M-2N=9,求多项式(2S+9M)-[-(2T-6N)]的值

问题描述:

已知S+T=21,3M-2N=9,求多项式(2S+9M)-[-(2T-6N)]的值

(2S+9M)-[-(2T-6N)]
=2S+9M+2T-6N
=2S+2T+9M-6N
=2(S+T)+3(3M-2N)
=2*21+3*9
=42+27
=69

首先将式子去掉括号,然后变形,得到
(2S+9M)-[-(2T-6N)]
=2S+9M+2T-6N
=2S+2T+9M-6N
=2(S+T)+3(3M-2N)
=2*21+3*9
=42+27
=69