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Algebra · real student question

The relation y = a x^b is non-linear. Change it into linear form Y = mX + c and state Y, X, m and c.

Question

The power law

y=a xby=a\,x^{b}

is non-linear. Reduce it to the linear form Y=mX+cY=mX+c and identify YY, XX, the gradient mm and the intercept cc.

Step-by-step solution

  1. Spot where the unknown constant sits. Here bb is an exponent on the variable xx, not on a constant. That difference decides the transformation: a power law needs logs on both sides and both variables, whereas an exponential law y=abxy=ab^{x} only needs a log on yy.

  2. Take logarithms of both sides.

    log⁡y=log⁡ ⁣(a xb)\log y=\log\!\left(a\,x^{b}\right)

  3. Split the product. By log⁡(MN)=log⁡M+log⁡N\log(MN)=\log M+\log N:

    log⁡y=log⁡a+log⁡ ⁣(xb)\log y=\log a+\log\!\left(x^{b}\right)

  4. Bring the power down. By log⁡(xb)=blog⁡x\log(x^{b})=b\log x:

    log⁡y=blog⁡x+log⁡a\log y=b\log x+\log a

  5. Match against Y=mX+cY=mX+c.

    log⁡y⏟Y=b⏟mlog⁡x⏟X+log⁡a⏟c\underbrace{\log y}_{Y}=\underbrace{b}_{m}\underbrace{\log x}_{X}+\underbrace{\log a}_{c}

    Plotting log⁡y\log y against log⁡x\log x - a log-log plot - gives a straight line whose gradient is the exponent bb and whose intercept is log⁡a\log a.

  6. Verify with numbers. Take a=3,b=2,x=4a=3,b=2,x=4, so y=3⋅16=48y=3\cdot 16=48. Then log⁡1048=1.6812\log_{10}48=1.6812, while blog⁡104+log⁡103=2(0.6021)+0.4771=1.6812b\log_{10}4+\log_{10}3=2(0.6021)+0.4771=1.6812 ✓. Recovering the constants: b=mb=m and a=10ca=10^{c}.

Answer

log⁡y=blog⁡x+log⁡a,Y=log⁡y, X=log⁡x, m=b, c=log⁡a\log y=b\log x+\log a,\qquad Y=\log y,\ X=\log x,\ m=b,\ c=\log a

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