已知:a+b=3x²－5x＋4,a-c=3x²+2x－5.求：当x=4时,b+c的值等于多少?
问题描述：
已知:a+b=3x²－5x＋4,a-c=3x²+2x－5.求：当x=4时,b+c的值等于多少?
答
a+b-(a-c)=a+b-c+c=b+c
b+c
=(3x²－5x＋4)-(3x²+2x－5)
=3x²-5x+4-3x²-2x+5
=-7x+9
=-7x4+9
=-28+9
=-19