即\s\up8(→(→)=\s\up8(→(→)+\s\up8(→(→),因此点P与点A,B,C共面.
1.已知空间四边形ABCD中,\s\up8(→(→)=a,\s\up8(→(→)=b,\s\up8(→(→)=c,则\s\up8(→(→)=( )
A.a+b-c B.-a-b+c
C.-a+b+c D.-a+b-c
C [\s\up8(→(→)=\s\up8(→(→)+\s\up8(→(→)+\s\up8(→(→)=\s\up8(→(→)-\s\up8(→(→)+\s\up8(→(→)=-a+b+c.]
2.在下列条件中,使M与A,B,C一定共面的是( )
A.\s\up8(→(→)=2\s\up8(→(→)-\s\up8(→(→)-\s\up8(→(→)
B.\s\up8(→(→)=\s\up8(→(→)+\s\up8(→(→)+\s\up8(→(→)
C.\s\up8(→(→)+\s\up8(→(→)+\s\up8(→(→)=0
D.\s\up8(→(→)+\s\up8(→(→)+\s\up8(→(→)+\s\up8(→(→)=0
C [由MA+MB+MC=0得\s\up8(→(→)=-\s\up8(→(→)-\s\up8(→(→),故M,A,B,C四点共面.]
3.在三棱锥ABCD中,若△BCD是正三角形,E为其中心,则\s\up8(→(→)+\s\up8(→(→)-\s\up8(→(→)-\s\up8(→(→)化简的结果为________.
0 [延长DE交边BC于点F,则有\s\up8(→(→)+\s\up8(→(→)=\s\up8(→(→),\s\up8(→(→)+\s\up8(→(→)=\s\up8(→(→)+\s\up8(→(→)=\s\up8(→(→),故\s\up8(→(→)+\s\up8(→(→)-\s\up8(→(→)-\s\up8(→(→)=0.]
4.如图,在长方体ABCDA1B1C1D1中,下列各式运算结果为\s\up8(→(→)的是________(填序号).
①\s\up8(→(→)-\s\up8(→(→)-\s\up8(→(→);
②\s\up8(→(→)+\s\up8(→(→)-\s\up8(→(→);
③\s\up8(→(→)-\s\up8(→(→)-\s\up8(→(→);
④\s\up8(→(→)-\s\up8(→(→)+\s\up8(→(→).