`a. x^45 - x^43 = 5^42. ( 5^3 - 5 )`
`=> x^42.(x^3 - x) = 5^42.(5^3 - 5)`
`=> x = 5`
`g. 7^2x + 1 + 7^2x - 1 = 50.72021`
`=> 7^2x = 1800525`
`=> 2x =` $log_{7}$ `( 1800525 )`
`=> x =` $\frac{log7 (1800525)}{2}$
`i. (x^2+2x+1).(x^2+2x+3)=99`
`=> ( x + 1 )^2 [ x ( x + 2 ) +3 ] = 99`
`=> x^4 + 4x^3 + 8x^2 + 8x - 96 = 0`
`=> x^4 + 4x^3 + 8x^2 + 8x + 3 = 99`
`=> x = -4` hoặc `x = 2`
`k. (x^2+5x).(x^2+5x-2)=525`
`=> x ( x + 5 ) [ ( x ( x + 5 ) - 2 ] = 525`
`=> x^4 + 10x^3 + 23x^2 - 10x - 525 = 0`
`=> x^4 + 10x^3 + 23x^2 - 10x = 525`
`=> x = 1/2`