选修2-2 1.5.3 定积分的概念
一、选择题
1.定积分??1
3(-3)d x 等于( ) A .-6
B .6
C .-3
D .3
[答案] A [解析] 由积分的几何意义可知??1
3(-3)d x 表示由x =1,x =3,y =0及y =-3所围成的矩形面积的相反数,故??1
3(-3)d x =-6. 2.定积分??a
b f (x )d x 的大小( ) A .与f (x )和积分区间[a ,b ]有关,与ξi 的取法无关
B .与f (x )有关,与区间[a ,b ]以及ξi 的取法无关
C .与f (x )以及ξi 的取法有关,与区间[a ,b ]无关
D .与f (x )、区间[a ,b ]和ξi 的取法都有关
[答案] A
[解析] 由定积分定义及求曲边梯形面积的四个步骤知A 正确.
3.下列说法成立的个数是( )
①??a
b f (x )d x =∑i =1n
f (ξi )b -a n ②??a b f (x )d x 等于当n 趋近于+∞时,f (ξi )·b -a n
无限趋近的值 ③??a
b f (x )d x 等于当n 无限趋近于+∞时,∑i =1n f (ξi )b -a n 无限趋近的常数 ④??a b f (x )d x 可以是一个函数式子
A .1
B .2
C .3
D .4 [答案] A
[解析] 由??a
b f (x )d x 的定义及求法知仅③正确,其余不正确.故应选A. 4.已知??1
3f (x )d x =56,则( )
A.??1
2f (x )d x =28 B.??23f (x )d x =28 C.??122f (x )d x =56
D.??12f (x )d x +??2
3f (x )d x =56 [答案] D [解析] 由y =f (x ),x =1,x =3及y =0围成的曲边梯形可分拆成两个:由y =f (x ),x =1,x =2及y =0围成的曲边梯形知由y =f (x ),x =2,x =3及y =0围成的曲边梯形.
∴??1
3f (x )d x =??12f (x )d x +??23f (x )d x 即??12f (x )d x +??23f (x )d x =56.
故应选D.
5.已知??a b f (x )d x =6,则??a
b 6f (x )d x 等于( ) A .6
B .6(b -a )
C .36
D .不确定
[答案] C [解析] ∵??a
b f (x )d x =6, ∴在??a b 6f (x )d x 中曲边梯形上、下底长变为原来的6倍,由梯形面积公式,知??a b 6f (x )d x =6??a
b f (x )d x =36.故应选C. 6.设f (x )=????? x 2 (x ≥0),2x (x <0),则?
?1-1f (x )d x 的值是( )
[答案] D
[解析] 由定积分性质(3)求f (x )在区间[-1,1]上的定积分,可以通过求f (x )在区间[-1,0]与[0,1]上的定积分来实现,显然D 正确,故应选D.
7.下列命题不正确的是( )
A .若f (x )是连续的奇函数,则
B .若f (x )是连续的偶函数,则
C .若f (x )在[a ,b ]上连续且恒正,则??a
b f (x )d x >0 D .若f (x )在[a ,b )上连续且??a
b f (x )d x >0,则f (x )在[a ,b )上恒正