ab垂直
3*4+5cosa*(-4tana)=0
12-20sina=0
sina=3/5
α∈(0,派/2) cosa=4/5 tana=3/4
|(a+b)|=|(7,5cosa-4tana)|=√[49+(5cosa-4tana)²]=√50
cos(a+π/4)=cosacoaπ/4-sinasinπ/4=4/5*√2/2-3/5*√2/2=√2/10
望采纳~~~~~~
ab垂直
3*4+5cosa*(-4tana)=0
12-20sina=0
sina=3/5
α∈(0,派/2) cosa=4/5 tana=3/4
|(a+b)|=|(7,5cosa-4tana)|=√[49+(5cosa-4tana)²]=√50
cos(a+π/4)=cosacoaπ/4-sinasinπ/4=4/5*√2/2-3/5*√2/2=√2/10
望采纳~~~~~~