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因式分解 (12 10:8:29)

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因式分解 (12 10:8:29)
(X2+3X+2)(X2+7X+12)-120
 
(X+1)(X+2)(X+3)(X+4)+1
 
已a,b,c 满足a+b=8,ab+c2=16  求a+b+c的值
(X2+3X+2)(X2+7X+12)-120
=[x^2+(2+1)x+1*2][x^2+(3+4)x+3*4]-120
=(x+1)(x+2)(x+3)(x+4)-120
=[(x+2)(x+3)][(x+1)(x+4)]-120
=(x^2+5x+6)(x^2+5x+4)-120
=(x^2+5x)^2+10(x^2+5x)+24-120
=(x^2+5x)^2+10(x^2+5x)-96
=(x^2+5x)^2+[16+(-6)](x^2+5x)+(16)*(-6)
=(x^2+5x-6)(x^2+5x+16)
=(x-1)(x+6)(x^2+5x+16)
(X+1)(X+2)(X+3)(X+4)+1
=[(x+2)(x+3)][(x+1)(x+4)]+1
=(x^2+5x+6)(x^2+5x+4)+1
=(x^2+5x)^2+10(x^2+5x)+24+1
=(x^2+5x)^2+10(x^2+5x)+25
=(x^2+5x)^2+2*(x^2+5x)*5 +5^2
=(x^2+5x+5)^2
下面一题看不懂.