十字相乘法，过程 十字相乘法的计算步骤
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\u7136\u540e\u4f7fa1c2+a2c1 =1*2+1*3 = b =6\u3002
\u6240\u4ee5\u516c\u5f0f\u53ef\u4ee5\u6574\u7406\u4e3a\uff08x +3\uff09\uff082x+1\uff09 =0
\u7ed3\u679c\u4e3ax =-1\u548cx =-1/2\u3002
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\u5219\uff1a[A*M+B*(S\uff0dM\uff09]/S=C
A*M/S+B*(S-M)/S=C
M/S=(C-B)/(A-B)
1-M/S=(A-C)/(A-B)
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A ^C-B
^C
B^ A-C
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x2=5
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6x²-5x-25=0
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\u6240\u4ee5
x1=5/2
x2=-5/3
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(2x-9y)(7x-2y)
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x1=2a+b
x2=a-b
第一步将二次幂的系数化为1，然后只要将常数项分解为A*B，将一次幂的系数凑为A+B的和，就好了。
这道题，二次幂系数已经为1了，所以我们先看常数项-8，很明细-8=（-1）*8=1*（-8）=2*（-4）=（-2)*4，再观察一次幂的系数为-2，所以我们选择2、-4这两个。
式子就变成了（a+2）（a-4）≥0了，后面就简单了，相信你能解开。
解：(a+4)(a-2)>=0
a>=2或a<=-4
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