1.(1)(a+5) (3a+2)
(2)(n-2)
(3)(4a+4b)cm (a+b)²cm²
(4)(1+a%)m
(5)a/2π
2.(答案不唯一)
(1)半径分别为R和r的两圆面积之差为πR²-πr²
(2)钢笔和铅笔的单价分别为x元,y元,则8枝钢笔和9枝笔的价钱为(8x+9y)元
(3)四个人的数学成绩分别为a,b,c,d,则这四个人的平均分为(a+b+c+d)/4
(4)a与b的差的12倍
(5)x的90%与y的110%的和
(6)a,b两数的与差的极
3.提示:(1)4xy+1/2. (2)-5x³-3x². (3)-8m²+8mn. (4)4ab. (5)a+5.
(6)9x-14. (7)4ab-5b². (8)a+3b.
4.解:①(3x²-1)/2. ②+1,()²,×2.填表由左至右,由上至下依次为13,11/2,1,-1/2,1,11/2,13;8,2,0,2,8,18,32.
5.提示:(1)0. (2)-2. (3)-7.5. (4)36.
6.解:(1)3/2 m-(5/2 m-1)+3(4-m)=3/2 m- 5/2 m+1+12-3m=-4m+13. 当m=-3时,原式=-4×(-3)+13=12+13=25.
(2)5a²-【3a-(2a-3)+4a²】=5a²-(3a-2a+3+4a²)=5a²-a-3-4a²=a²-a-3.当a=-2时,原式=(-2)²-(-2)-3=4+2-3=3.
(3)5(3a²b-ab²)-4(-ab²+3a²b)=15a²b-5ab²+4ab²-12a²b=3a²b-ab².当a=1/2,b=-1/3 时,原式=3×(1/2)²×(-1/3)-1/2×(-1/3)^2=3×1/4×(-1/3)-1/2×1/9=-1/4-1/18=-11/36.
(4)4xy-【(x²+5xy-y²)-(x²+3xy-2y²)】=4xy-(x²+5xy-y²-x²-3xy+2y²)=4xy-(y²+2xy)=4xy+y²-2xy=2xy-y².当x=-1/4,y=-1/2时,原式=2×(-1/4)×(-1/2)-(-1/2)²=1/4-1/4=0. (5)abc-【2ab-(3abc-bc)+4abc】=abc-(2ab-3abc+bc+4abc)=abc-2ab-bc-abc=-2ab-bc.当a=2,b=-1/2,c=-1时,原式=-2×2(-1/2)-(-1/2)×(-1)=2-1/2=3/2.
7.解:原式=4(t²-t-1).当t=-1/2 时,原式=4【(-1/2)^2-(-1/2)-1】=-1.
8.解:πab/2+π(a/2)^2=πab/2+(πa^2)/4 (m^2 ).
9.解:(1)(60-5.5t)升.
(2)当t=4.75时,60-5.5t=60-5.5×4.75=60-26.125=33.875(L);当t=6.5时,60-5.5t=60-5.5×6.5=24.25(L);当t=9时,60-5.5t=60-5.5×9=10.5(L).
10.(1)4 7 10 (2)(3n+1)
11.提示:3次,6次,(n(n-1))/2 次.
12.提示:1/2π m.
13.解:本题答案不唯一,如-5x,-4x-4等.
14.解:本题答案不唯一,如丨a丨+1,a²+丨a丨+1等.
15.解:本题答案不唯一,如a²+2ab和b²-2ab.
16.解:当b>0时,a+b>a-b;当b=0时,a+b=a-b;当b>0时,a+b<a-b.
17.解:由题意知5a+3b=-4,则2(a+b)+4(2a+b)=2a+2b+8a+4b=10a+6a=2(5a+3b)=2×(-4)=-8.
18.解:相信.理由如下:设所想的数为x,依题意得(2x+8)/4-x/2=x/2+2-x/2=2.