Sin20x+10cos10x=0 (9.4:9.7) -?

Question

Sin20x+10cos10x=0 (9.4:9.7) -?

Answer 1

2sin10xcos10x+10cos10x=0
2cos10x(sin10x+5)=0
cos10x=0⇒10x=π/2+πn⇒x=π/20+πn/10
sin10x=-5∉[-1;1]