Тригонометрия.2 sin(pi-a)-3cos(pi\2-a),если sin a= 0.4Найти Ctg a,если cos a = 3\5,при...

1) $2\sin( \pi-\alpha )-3\cos(\frac\pi2- \alpha )=2\sin \alpha -3\sin \alpha =-\sin \alpha =-0.4$
2) $\sin \alpha =-\sqrt{1-\cos^2 \alpha }=-\sqrt{1-(\frac35)^2}=-\frac45\\ ctg \alpha =\frac{\cos \alpha }{\sin \alpha }=-\frac{\frac35}{\frac45}=-\frac{3*5}{4*5}=-\frac34$
3) $9\cos^2 \alpha +5\sin^2 \alpha =9(1-\sin^2 \alpha )+5\sin^2 \alpha =\\ =9-9\sin^2 \alpha +5\sin^2 \alpha =9-4\sin^2 \alpha =\\ 9-4*(-0.6)^2=9-1.44=7.56$
4) $\frac{2\sin34\cos34}{\sin64}=\frac{\sin(2*34)}{\sin64}=\frac{\sin64}{\sin64}=1$