Question

必修四数学三角恒等变换已知ABC是△ABC的三个内角,若cosA＜0,且cos2A-3sinA+1=0,责sin(C-A

Answer 1

sina+cosa=(1-√5)/2

两边平方

(sina+cosa)²=(6-2√5)/4

即 1+2sinacosa=(3-√5)/2

∴ 2sinacosa=(1-√5)/2