2. Solution: 1/4*c? *tanA= 1/4*a? Tambu, get an A? /c? =
TanA/tanB, so b=c is an isosceles triangle, and a=b=c is an equilateral triangle.
3. Solution: By
Two angles sum formula sin(A+B)
=
sinAcosB+cosAsinB
And the angle doubling formula Sin2A=2SinA*CosA.
cos2a = COSA 2-Sina 2 = 1-2 Sina 2 = 2 COSA 2- 1。
F(x) = sin (2x+30) = sin {2 (x+15)}, so the period of f (x) is π.
When x+ 15 = π/4+nπ, y takes the maximum value, so the value range of x is π/6+nπ.
You work hard and everyone is happy. I hope it helps you.