1) ĐK: `x \ne π/10 +kπ/5 ; x \ne π/4 +kπ/2`
`tan 5x .tan2x=1`
`<=> tan5x=cot2x`
`<=>tan5x=tan(π/2-2x)`
`<=>5x=π/2-2x+kπ`
`<=>x=π/14+kπ/7 (k in ZZ)`
2) ĐK: `x \ne π/8+kπ/4 ; x \ne π/18 + kπ/9`
`tan4x .tan9x=1`
`<=> tan4x=cot9x`
`<=>tan4x=tan(π/2-9x)`
`<=>4x=π/2-9x+kπ`
`<=>13x=π/2+kπ`
`<=>x=π/26+kπ/13\ (k in ZZ)`