函数f(x)=3sin(x+10°)+5sin(x+70°)的最大值是(  ) A.5.5 B.6.5 C.7 D.8

问题描述:

函数f(x)=3sin(x+10°)+5sin(x+70°)的最大值是(  )
A. 5.5
B. 6.5
C. 7
D. 8

f(x)=3sin(x+100)+5sin[(x+100)+600]
=3sin(x+100)+5sin(x+100)cos60°+5cos(x+100)sin60°
=

11
2
sin(x+10°)+
5
3
2
cos(x+10°)
=
(
11
2
)
2
+(
5
3
2
)
2
sin(x+100+∅),其中tan∅=
5
3
11

(
11
2
)
2
+(
5
3
2
)
2
=7
故选C.