arctan1/4x-90-arctan1/7x-arctan0.2x-arctan0.05x=-180
    arctan1/4w-90-arctan1/7w-arctan0.2w-arctan0.05w=-180
    arctan0.2x+arctan0.05x=90
    arctan( 1/4x)-90-arctan (1/7x)-arctan(0.2x)-arctan (0.05x)=180
    x*x(1+0.04x*x)(1+0.0025x*x)=36
    x^2-2x-3=0
    x2-2x-3=0
    x2=4
    0 = x^6-17*x^4-12*x^3+49*x^2+48*x-13
    3.5（1-2x）=2. 1
    (x^2)/16+x-81/36=0
    2/3x÷1/4x=12
    2/7 x+1/2 x等于11
    18+9*0.4=1.8x
    18+0.4x=1.8x
    18+4x=1.8x
    x^2=（1+x）*（1+x）
    2-632*x^（5/3）=x
    15/133.58+x/1000+(62-x)/1650=0.136
    2x+2000÷7x-2000＝5÷7

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