若(x²+ax+5)(x²-3x+b)的结果不含x²与x³项,试求a b的值
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若(x²+ax+5)(x²-3x+b)的结果不含x²与x³项,试求a b的值
若(x²+ax+5)(x²-3x+b)的结果不含x²与x³项,试求a b的值
若(x²+ax+5)(x²-3x+b)的结果不含x²与x³项,试求a b的值
最原始的办法
(x²+ax+5)(x²-3x+b)
=x^4-3x^3+bx^2+ax^3-3ax^2+abx+5x^2-15x+5b
=x^4+(a-3)x^3+(b-3a+5)x^2+(ab-15)x+5b
不含x²与x³项
只需a-3=0
b-3a+5=0
得a=3 ,b=4
若(x²+ax+5)(x²-3x+b)的结果不含x²与x³项,试求a b的值
(x²+ax+5)(x²-3x+b)=x⁴+(a-3)x³+(5-3a+b)x²+(15+ab)x+5b
∵不含x²和x³项,∴a-3=0,即有a=3;5-3a+b=5-9+b=-4+b=0,故b=4.