`color{#8FBC8F}{~} color{#C1FFC1}{b} color{#B4EEB4}{u} color{#9BCD9B}{i} color{#698B69}{g} color{#2E8B57}{i} color{#54FF9F}{a} color{#4EEE94}{p} color{#43CD80}{h} color{#98FB98}{o} color{#008B45}{n} color{#00FF00}{g} color{#00EE00}{9} color{#00CD00}{9} color{#ADFF2F}{9} color{#228B22}{~}`

`5cos2x-12sinx-13=0`

`<=> 5(1-2sin^2x0-12sinx)-13=0`

`<=> 5-10sin^2x-12sinx-13=0`

`<=> -10sin^2x-12sinx-8=0`

`<=> 5sin^2x+6sinx+4=0`

Đặt `t=sinx (-1 le x le 1)`

`5t^2+6t+4=0`

`PT` có `a=5,b=6,c=4`

`Delta = 6^2-4.5.4=-44<0`

Do `Delta <0` nên phương trình đã cho vô nghiệm.