Question

👀

求算法

10. 設日,a,b為實數且 cos 29
-
3
,若 sin'9和cos_日為方程式x + CN+b=0之兩根,則
5
B
a+b = ?
19
21
29
29
(B)
(A
(A)
(C
(C)
0
( (D)
25
25
25
25

Answer 1

S

cos(2x) = cos^2(x) – sin^2(x)
= 1 – 2sin^2(x)= 2cos^2(x) – 1

cos^2(x) =4/5，sin^2(x) =1/5
根與係數關係知
-a=sin^2(x) +cos^2(x) =1，a=-1
b=sin^2(x) ·cos^2(x) =4/25
(-1)+(4/25)= -(21/25)