初中数学.十字相乘法我不会谁教我!! 今天数学老师教十字相乘法不会,我一头雾水,谁拯救拯救我怎么解...
\u8c01\u80fd\u6559\u4e0b\u6211\u521d\u4e2d\u6570\u5b66\u7684\u201c\u5341\u5b57\u76f8\u4e58\u6cd5\u201d\u5341\u5b57\u76f8\u4e58\u6cd5\u2014\u2014\u501f\u52a9\u753b\u5341\u5b57\u4ea4\u53c9\u7ebf\u5206\u89e3\u7cfb\u6570\uff0c\u4ece\u800c\u628a\u4e8c\u6b21\u4e09\u9879\u5f0f\u5206\u89e3\u56e0\u5f0f\u7684\u65b9\u6cd5\u53eb\u505a\u5341\u5b57\u76f8\u4e58\u6cd5\u3002
\u5341\u5b57\u76f8\u4e58\u6cd5\u662f\u4e8c\u6b21\u4e09\u9879\u5f0f\u5206\u89e3\u56e0\u5f0f\u7684\u4e00\u79cd\u5e38\u7528\u65b9\u6cd5\uff0c\u5b83\u662f\u5148\u5c06\u4e8c\u6b21\u4e09\u9879\u5f0f
\u7684\u4e8c\u6b21\u9879\u7cfb\u6570a\u53ca\u5e38\u6570\u9879c\u90fd\u5206\u89e3\u4e3a\u4e24\u4e2a\u56e0\u6570\u7684\u4e58\u79ef\uff08\u4e00\u822c\u4f1a\u6709\u51e0\u79cd\u4e0d\u540c\u7684\u5206\u6cd5\uff09
\u7136\u540e\u6309\u659c\u7ebf\u4ea4\u53c9\u76f8\u4e58\u3001\u518d\u76f8\u52a0\uff0c\u82e5\u6709
\uff0c\u5219\u6709
\uff0c\u5426\u5219\uff0c\u9700\u4ea4\u6362
\u7684\u4f4d\u7f6e\u518d\u8bd5\uff0c\u82e5\u4ecd\u4e0d\u884c\uff0c\u518d\u6362\u53e6\u4e00\u7ec4\uff0c\u7528\u540c\u6837\u7684\u65b9\u6cd5\u8bd5\u9a8c\uff0c\u76f4\u5230\u627e\u5230\u5408\u9002\u7684\u4e3a\u6b62\u3002
\u5728\u6211\u4eec\u505a\u56e0\u5f0f\u5206\u89e3\u9898\u65f6\uff0c\u53ef\u4ee5\u53c2\u7167\u4e0b\u9762\u7684\u53e3\u8bc0\uff1a
\u9996\u5148\u63d0\u53d6\u516c\u56e0\u5f0f\uff0c\u7136\u540e\u8003\u8651\u7528\u516c\u5f0f\uff1b
\u5341\u5b57\u76f8\u4e58\u8bd5\u4e00\u8bd5\uff0c\u5206\u7ec4\u5206\u5f97\u8981\u5408\u9002\uff1b
\u56db\u79cd\u65b9\u6cd5\u53cd\u590d\u8bd5\uff0c\u6700\u540e\u987b\u662f\u8fde\u4e58\u5f0f\u3002
\u5341\u5b57\u76f8\u4e58\u6cd5\u89e3\u9898\u5b9e\u4f8b\uff1a
1)\u3001
\u7528\u5341\u5b57\u76f8\u4e58\u6cd5\u89e3\u4e00\u4e9b\u7b80\u5355\u5e38\u89c1\u7684\u9898\u76ee
\u4f8b1\u628am²+4m-12\u5206\u89e3\u56e0\u5f0f
\u5206\u6790\uff1a\u672c\u9898\u4e2d\u5e38\u6570\u9879-12\u53ef\u4ee5\u5206\u4e3a-1\u00d712\uff0c-2\u00d76\uff0c-3\u00d74\uff0c-4\u00d73\uff0c-6\u00d72\uff0c-12\u00d71\u5f53-12\u5206\u6210-2\u00d76\u65f6\uff0c\u624d\u7b26\u5408\u672c\u9898
\u89e3\uff1a\u56e0\u4e3a
1
-2
1
\u2573
6
\u6240\u4ee5m²+4m-12=\uff08m-2\uff09\uff08m+6\uff09
\u4f8b2\u628a5x²+6x-8\u5206\u89e3\u56e0\u5f0f
\u5206\u6790\uff1a\u672c\u9898\u4e2d\u76845\u53ef\u5206\u4e3a1\u00d75,-8\u53ef\u5206\u4e3a-1\u00d78\uff0c-2\u00d74\uff0c-4\u00d72\uff0c-8\u00d71\u3002\u5f53\u4e8c\u6b21\u9879\u7cfb\u6570\u5206\u4e3a1\u00d75\uff0c\u5e38\u6570\u9879\u5206\u4e3a-4\u00d72\u65f6\uff0c\u624d\u7b26\u5408\u672c\u9898
\u89e3\uff1a
\u56e0\u4e3a
1
2
5
\u2573
-4
\u6240\u4ee55x²+6x-8=\uff08x+2\uff09\uff085x-4\uff09
\u4f8b3\u89e3\u65b9\u7a0bx²-8x+15=0
\u5206\u6790\uff1a\u628ax²-8x+15\u770b\u6210\u5173\u4e8ex\u7684\u4e00\u4e2a\u4e8c\u6b21\u4e09\u9879\u5f0f\uff0c\u521915\u53ef\u5206\u62101\u00d715\uff0c3\u00d75\u3002
\u89e3\uff1a
\u56e0\u4e3a
1
-3
1
\u2573
-5
\u6240\u4ee5\u539f\u65b9\u7a0b\u53ef\u53d8\u5f62\uff08x-3\uff09\uff08x-5\uff09=0
\u6240\u4ee5x1=3
x2=5
\u4f8b4\u3001\u89e3\u65b9\u7a0b
6x²-5x-25=0
\u5206\u6790\uff1a\u628a6x²-5x-25\u770b\u6210\u4e00\u4e2a\u5173\u4e8ex\u7684\u4e8c\u6b21\u4e09\u9879\u5f0f\uff0c\u52196\u53ef\u4ee5\u5206\u4e3a1\u00d76\uff0c2\u00d73\uff0c-25\u53ef\u4ee5\u5206\u6210-1\u00d725\uff0c-5\u00d75\uff0c-25\u00d71\u3002
\u89e3\uff1a
\u56e0\u4e3a
2
-5
3
\u2573
5
\u6240\u4ee5
\u539f\u65b9\u7a0b\u53ef\u53d8\u5f62\u6210\uff082x-5\uff09\uff083x+5\uff09=0
\u6240\u4ee5
x1=5/2
x2=-5/3
2)\u3001\u7528\u5341\u5b57\u76f8\u4e58\u6cd5\u89e3\u4e00\u4e9b\u6bd4\u8f83\u96be\u7684\u9898\u76ee
\u4f8b5\u628a14x²-67xy+18y²\u5206\u89e3\u56e0\u5f0f
\u5206\u6790\uff1a\u628a14x²-67xy+18y²\u770b\u6210\u662f\u4e00\u4e2a\u5173\u4e8ex\u7684\u4e8c\u6b21\u4e09\u9879\u5f0f,\u521914\u53ef\u5206\u4e3a1\u00d714,2\u00d77,
18y²\u53ef\u5206\u4e3ay.18y
,
2y.9y
,
3y.6y
\u89e3:
\u56e0\u4e3a
2
-9y
7
\u2573
-2y
\u6240\u4ee5
14x²-67xy+18y²=
(2x-9y)(7x-2y)
\u4f8b6
\u628a10x²-27xy-28y²-x+25y-3\u5206\u89e3\u56e0\u5f0f
\u5206\u6790\uff1a\u5728\u672c\u9898\u4e2d\uff0c\u8981\u628a\u8fd9\u4e2a\u591a\u9879\u5f0f\u6574\u7406\u6210\u4e8c\u6b21\u4e09\u9879\u5f0f\u7684\u5f62\u5f0f
\u89e3\u6cd5\u4e00\u300110x²-27xy-28y²-x+25y-3
=10x²-\uff0827y+1\uff09x
-\uff0828y²-25y+3\uff09
4y
-3
7y
\u2573
-1
=10x²-\uff0827y+1\uff09x
-\uff084y-3\uff09\uff087y
-1\uff09
=[2x
-\uff087y
-1\uff09][5x
+\uff084y
-3\uff09]
2
-\uff087y
\u2013
1\uff09
5
\u2573
4y
-
3
=\uff082x
-7y
+1\uff09\uff085x
+4y
-3\uff09
\u8bf4\u660e\uff1a\u5728\u672c\u9898\u4e2d\u5148\u628a28y²-25y+3\u7528\u5341\u5b57\u76f8\u4e58\u6cd5\u5206\u89e3\u4e3a\uff084y-3\uff09\uff087y
-1\uff09\uff0c\u518d\u7528\u5341\u5b57\u76f8\u4e58\u6cd5\u628a10x²-\uff0827y+1\uff09x
-\uff084y-3\uff09\uff087y
-1\uff09\u5206\u89e3\u4e3a[2x
-\uff087y
-1\uff09][5x
+\uff084y
-3\uff09]
\u89e3\u6cd5\u4e8c\u300110x²-27xy-28y²-x+25y-3
=\uff082x
-7y\uff09\uff085x
+4y\uff09-\uff08x
-25y\uff09-
3
2
-7y
=[\uff082x
-7y\uff09+1]
[\uff085x
-4y\uff09-3]
5
\u2573
4y
=\uff082x
-7y+1\uff09\uff085x
-4y
-3\uff09
2
x
-7y
1
5
x
-
4y
\u2573
-3
\u8bf4\u660e:\u5728\u672c\u9898\u4e2d\u5148\u628a10x²-27xy-28y²\u7528\u5341\u5b57\u76f8\u4e58\u6cd5\u5206\u89e3\u4e3a\uff082x
-7y\uff09\uff085x
+4y\uff09,\u518d\u628a\uff082x
-7y\uff09\uff085x
+4y\uff09-\uff08x
-25y\uff09-
3\u7528\u5341\u5b57\u76f8\u4e58\u6cd5\u5206\u89e3\u4e3a[\uff082x
-7y\uff09+1]
[\uff085x
-4y\uff09-3].
\u4f8b7\uff1a\u89e3\u5173\u4e8ex\u65b9\u7a0b\uff1ax²-
3ax
+
2a²\u2013ab
-b²=0
\u5206\u6790\uff1a2a²\u2013ab-b²\u53ef\u4ee5\u7528\u5341\u5b57\u76f8\u4e58\u6cd5\u8fdb\u884c\u56e0\u5f0f\u5206\u89e3
\u89e3\uff1ax²-
3ax
+
2a²\u2013ab
-b²=0
x²-
3ax
+\uff082a²\u2013ab
-
b²\uff09=0
x²-
3ax
+\uff082a+b\uff09\uff08a-b\uff09=0
1
-b
2
\u2573
+b
[x-\uff082a+b\uff09][
x-\uff08a-b\uff09]=0
1
-\uff082a+b\uff09
1
\u2573
-\uff08a-b\uff09
\u6240\u4ee5
x1=2a+b
x2=a-b
\u662f\u9ad8\u4e2d\u7684\u5185\u5bb9\u5417\uff1f\u5341\u5b57\u76f8\u4e58\u6cd5\u2014\u2014\u501f\u52a9\u753b\u5341\u5b57\u4ea4\u53c9\u7ebf\u5206\u89e3\u7cfb\u6570\uff0c\u4ece\u800c\u628a\u4e8c\u6b21\u4e09\u9879\u5f0f\u5206\u89e3\u56e0\u5f0f\u7684\u65b9\u6cd5\u53eb\u505a\u5341\u5b57\u76f8\u4e58\u6cd5\u3002
\u5341\u5b57\u76f8\u4e58\u6cd5\u662f\u4e8c\u6b21\u4e09\u9879\u5f0f\u5206\u89e3\u56e0\u5f0f\u7684\u4e00\u79cd\u5e38\u7528\u65b9\u6cd5\uff0c\u5b83\u662f\u5148\u5c06\u4e8c\u6b21\u4e09\u9879\u5f0f \u7684\u4e8c\u6b21\u9879\u7cfb\u6570a\u53ca\u5e38\u6570\u9879c\u90fd\u5206\u89e3\u4e3a\u4e24\u4e2a\u56e0\u6570\u7684\u4e58\u79ef\uff08\u4e00\u822c\u4f1a\u6709\u51e0\u79cd\u4e0d\u540c\u7684\u5206\u6cd5\uff09
\u7136\u540e\u6309\u659c\u7ebf\u4ea4\u53c9\u76f8\u4e58\u3001\u518d\u76f8\u52a0\uff0c\u82e5\u6709 \uff0c\u5219\u6709 \uff0c\u5426\u5219\uff0c\u9700\u4ea4\u6362 \u7684\u4f4d\u7f6e\u518d\u8bd5\uff0c\u82e5\u4ecd\u4e0d\u884c\uff0c\u518d\u6362\u53e6\u4e00\u7ec4\uff0c\u7528\u540c\u6837\u7684\u65b9\u6cd5\u8bd5\u9a8c\uff0c\u76f4\u5230\u627e\u5230\u5408\u9002\u7684\u4e3a\u6b62\u3002
.\u56e0\u5f0f\u5206\u89e3\u7684\u4e00\u822c\u6b65\u9aa4
\uff081\uff09 \u5982\u679c\u591a\u9879\u5f0f\u7684\u5404\u9879\u6709\u516c\u56e0\u5f0f\u65f6\uff0c\u5e94\u5148\u63d0\u53d6\u516c\u56e0\u5f0f\uff1b
\uff082\uff09 \u5982\u679c\u591a\u9879\u5f0f\u7684\u5404\u9879\u6ca1\u6709\u516c\u56e0\u5f0f\uff0c\u5219\u8003\u8651\u662f\u5426\u80fd\u7528\u516c\u5f0f\u6cd5\u6765\u5206\u89e3\uff1b
\uff083\uff09 \u5bf9\u4e8e\u4e8c\u6b21\u4e09\u9879\u5f0f\u7684\u56e0\u5f0f\u5206\u89e3\uff0c\u53ef\u8003\u8651\u7528\u5341\u5b57\u76f8\u4e58\u6cd5\u5206\u89e3\uff1b
\uff084\uff09 \u5bf9\u4e8e\u591a\u4e8e\u4e09\u9879\u7684\u591a\u9879\u5f0f\uff0c\u4e00\u822c\u5e94\u8003\u8651\u4f7f\u7528\u5206\u7ec4\u5206\u89e3\u6cd5\u8fdb\u884c\u3002
\u5728\u8fdb\u884c\u56e0\u5f0f\u5206\u89e3\u65f6\uff0c\u8981\u7ed3\u5408\u9898\u76ee\u7684\u5f62\u5f0f\u548c\u7279\u70b9\u6765\u9009\u62e9\u786e\u5b9a\u91c7\u7528\u54ea\u79cd\u65b9\u6cd5\u3002\u4ee5\u4e0a\u8fd9\u56db\u79cd\u65b9\u6cd5\u662f\u5f7c\u6b64\u6709\u8054\u7cfb\u7684\uff0c\u5e76\u4e0d\u662f\u4e00\u79cd\u7c7b\u578b\u7684\u591a\u9879\u5f0f\u5c31\u53ea\u80fd\u7528\u4e00\u79cd\u65b9\u6cd5\u6765\u5206\u89e3\u56e0\u5f0f\uff0c\u8981\u5b66\u4f1a\u5177\u4f53\u95ee\u9898\u5177\u4f53\u5206\u6790\u3002
\u5728\u6211\u4eec\u505a\u9898\u65f6\uff0c\u53ef\u4ee5\u53c2\u7167\u4e0b\u9762\u7684\u53e3\u8bc0\uff1a
\u9996\u5148\u63d0\u53d6\u516c\u56e0\u5f0f\uff0c\u7136\u540e\u8003\u8651\u7528\u516c\u5f0f\uff1b
\u5341\u5b57\u76f8\u4e58\u8bd5\u4e00\u8bd5\uff0c\u5206\u7ec4\u5206\u5f97\u8981\u5408\u9002\uff1b
\u56db\u79cd\u65b9\u6cd5\u53cd\u590d\u8bd5\uff0c\u6700\u540e\u987b\u662f\u8fde\u4e58\u5f0f\u3002
常数项分解为2个数相乘,分解的这两个数之和为一次项系数.
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