先化简；再求值。
\u5148\u5316\u7b80\uff0c\u518d\u6c42\u503c1. \uff082a-b\uff092-\uff08a+1-b\uff09\uff08a+1+b\uff09+\uff08a+1\uff092=\uff082a-b\uff092-(a+1)2+b2+\uff08a+1\uff092=\uff082a-b\uff092+b2=13
2. \uff08a+b\uff09\uff08a-2b\uff09-\uff08a+2b\uff09\uff08a-b\uff09=a2-ab-2b2-a2-ab+2b2=-2ab=4
U=I\uff08R1+R2+R3\uff09=2.3*(12.9+18.5+18.6)=115
1、3a的三次方--a的平方+a-1)—(4a的三次方+2a的平方—3a—6)=(3a^3-a^2+a-1)-(4a^3+2a^2-3a-6)
=3a^3-a^2+a-1-4a^3-2a^2+3a+6
=-a^3-3a^2+4a+5
将a=-1代入上式，得-(-1)^3-3(-1)^2+4(-1)+5=1-3-4+5=-1
2、-x的平方-二分之一［3y的平方-2(x的平方-y的平方)+6］
=-x^2-1/2*[3y^2-2x^2+2y^2+6]
=-x^2-3y^2/2+x^2-y^2-3
=-5y^2/2-3
将y=-1/2代入上式，得-5*(-1/2)^2/2-3=-5*1/4÷2-3=-5/8-3=-29/8
1，3a的三次方--a的平方+a-1)—(4a的三次方+2a的平方—3a—6)
=3a³-a²+a-1-4a³-2a²+3a+6
=a³-3a²+4a+5
=(-1)³-3(-1)²+4*(-1)+5
=1-3-4+5
=-1
2、-x的平方-二分之一［3y的平方-2(x的平方-y的平方)+6］
=-x²-1/2[3y²-2(x²-y²)+6]
=-x²-1/2(3y²-2x²+y²+6)
=-x²-1/2(4y²-2x²+6)
=-x²-2y²+x²-3
=-2y²-3
=-2(-1/2)²-3
=-1/2-3
=-3.5
第一题：
(3a³-a²+a-1)—(4a³+2a²-3a-6)
=3a³-a²+a-1—4a³-2a²+3a+6
=-a³-3a²+4a+5 (将a=-1代入)
=-(-1)³-3(-1)²+4*(-1)+5
=1-3-4+5
=-1
第二题：
-x²-1/2[3y²-2(x²-y²)+6］
=-x²-3/2y²+(x²-y²)-3
=-x²-3/2y²+x²-y²-3
=-5/2y²-3 (将y=-1/2代入)
=-5/2*(-1/2)²-3
=-5/8-3
=-29/8
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