x^2-10xy=-100,y^2-3xy=-40,求x^2-7xy-y^2及x^2-16xy+2y^2的值

1、原式=(x+1)/(x-3)(x+2)+3/[5(x-1)(x+2)]-(x-2)/[(x-1)(x-3)]
=[5(x+1)(x-1)+3(x-3)-(x-2)(x+2)]/[5(x-1)(x+2)(x-3)]
=(5x^2-5+3x-9-x^2+4)/[5(x-1)(x+2)(x-3)]=(4x^2+3x-10)/[5(x-1)(x+2)(x-3)]
=(x+2)(4x-5)/[5(x-1)(x+2)(x-3)]=(4x-5)/[5(x-1)(x-3)]
2、原式=(x+a-x+a)/(x^2-a^2)-2a/(x^2+a^2)-4a^3/(x^4-a^4)
=2a(x^2+a^2-x^2+a^2)/(x^4-a^4)-4a^3/(x^4-a^4)=0
3、原式=(x+2-x+2)/(x^2-4)-4(x^2+4)-32/(x^4-16)
=4(x^2+4-X^2+4)/(x^4-16)-32/(x^4-16)=0

解析：2.B 中和反应中,酸与碱的质量无关系
3.A 剩下的三个选项均是混合物
4.B ,A中氯化钠的化学式不对,是NaCl,C中为Ca(OH)2 ,D中为烧碱或者火碱或者苛性钠
5.B
6.B
7.只要是含N,P,K的三种中的两种或者三种均为复合肥,答案是C
8.化合物各元素的化合价之和为0,计算得A
9.实验中,不小心溅在身上强碱,需用弱酸,我们常用的是硼酸,而非盐酸,选A
10.过氧化氢分解之后的可发生的为,水与氧化钙,生成氢氧化钙,氢氧化钙与氯化铜,共三个.选B

7x/12-x/4=x-(x/4*1/2)-10
x/3=x-x/8-10
x/3=7x/8-10
13x/24=10
x=240/13

(5-a)^2-10(a-5)-56
=(a-5)²-10(a-5)-56
=(a-5+4)(a-5-14)
=(a-1)(a-19)

主要就是考这个:
(a-b)^2 = a^2 -2*a*b + b^2
同类还有一个公式:
(a+b)^2 = a^2 +2*a*b + b^2
不过 n^2 = 0 则 n=0 也是一个知识点.
若设a=X-2,b=5,可得到:a^2-2*a*b+b^2=0
即 (a-b)^2 = 0
a - b = 0
X-2 - 5 = 0
X = 7
展开原式也不难：
x^2-4x+4 - 10x+20 + 25=0
x^2 - 14x - 49 =0
(x-7)^2=0
x-7=0
x=7

1原式=[5（a-b）]^2-2*5（a-b)*1+1^2
=[5(a-b)-1]^2=(5a-5b-1)^2
2原式=x^2-4x-12+16
=x^2-2*x*2+2^2
=(x-2)^2
3原式=【x^2-2x+1】^2
=(x-1)^4

2x(x-12)+22
=2x²-24x+22
=2(x²-12x+11)
=2(x-1)(x-11)
3(a-2b)^4-39(a-2b)^2+108
=3[(a-2b)^4-13(a-2b)²+36]
=3[(a-2b)²-4][(a-2b)²-9]
=3(a-2b+2)(a-2b-2)(a-2b+3)(a-2b-3)
x^2-10xy+25y^2-6x+30y+8
=(x-5y)²-6(x-5y)+8
=(x-5y-2)(x-5y-4)

x^2-x+1/4
=x²-2x×1/2+(1/2)²
=(x-1/2)²
x^2-6x+9
=x²-2×x×3+3²
=(x-3)²
25y^2-80y+64
=(5y)²-2×5y×8+8²
=(5y-8)²
(x-y)^2-10(x-y)+25
=[(x-y)-5]²
=(x-y-5)²
-4x^2+12xy-9y^2
=-(4x²-12xy-9y²)
=-(2x-3y)²
9x^4+12x^2y^2+4y^4
=(3x²)²+2×3x²×2y²+(2y²)²
=(3x²+2y²)²

4X-3Y-6Z=0,X+2Y-7Z=0
得 x=13z/5,y=11z/5
(5X^2+2Y^2-Z^2)/(2X^2-3Y^2-10Z^2)=-241074z^4/625

2X2+3X-X2+X+42=X2-10
4X=-52
X=-13
同学你抄错符号了吧.若果那个是负号,答案才是-13呢.

完全平方
(a-b)^2-10(a-b)+25
=[(a-b)-5]^2
=(a-b-5)^2

因为根据公式y^2-10y+25=(y-5)^2
y=x^2+x
(x^+x)^2-10(x^2+x)+25
=(x^2+x-5)^2

a^2b^2-10ab+25
=a^2b^2-2*5ab+5^2
=(ab-5)^2