5*sin(x)+2*cos(x)=4,5 |-2 cos(x)
5*sin(x)=4,5 -2 cos(x) |(...)^2
25 sin^2(x)=(4,5 -2 cos(x))^2
25 sin^2(x)=20.25 -18 cos(x) +4 cos^2(x)
sin^2(x) +cos^2(x)=1
sin^2(x) = 1-cos^2(x)
------------->
25(1-cos^2(x)) =20.25 -18 cos(x) +4 cos^2(x)
25 -25 cos^2(x) =20.25 -18 cos(x) +4 cos^2(x)
0= 29 cos^2(x) -18 cos(x) -4.45=0
Substtution: z= cos(x)
---------->
29 z^2 -18 z -4.45=0 ->PQ- formel
Resubstituieren nicht vergessen und Probe machen.