Algebra · real student question

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

Question

The relation

y(bx)=axy(b-x)=ax

is non-linear in xx and yy. 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. Diagnose the type of non-linearity. Nothing here is an exponent - the trouble is the product xyxy hidden in y(bx)=byxyy(b-x)=by-xy. Taking logs would only make that worse. When variables multiply each other, the fix is usually reciprocals.

  2. Divide by the product xyxy. Starting from y(bx)=axy(b-x)=ax and dividing both sides by xyxy (valid for x0x\neq 0, y0y\neq 0):

    bxx=ay\frac{b-x}{x}=\frac{a}{y}

  3. Split the left-hand fraction.

    bx1=ay\frac{b}{x}-1=\frac{a}{y}

    Both variables now appear only as 1/x1/x and 1/y1/y - that is the signal the transformation is working.

  4. Solve for 1/y1/y. Divide through by aa:

    1y=ba1x1a\frac{1}{y}=\frac{b}{a}\cdot\frac{1}{x}-\frac{1}{a}

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

    1yY=bam1xX+(1a)c\underbrace{\frac{1}{y}}_{Y}=\underbrace{\frac{b}{a}}_{m}\underbrace{\frac{1}{x}}_{X}+\underbrace{\left(-\frac{1}{a}\right)}_{c}

    So plot 1/y1/y against 1/x1/x. The intercept gives a=1/ca=-1/c and then b=mab=ma.

  6. Check with numbers. Take a=2,b=5,x=1a=2,b=5,x=1: the original gives y(51)=2y(5-1)=2, so y=0.5y=0.5 and 1/y=21/y=2. The linear form gives 52(1)12=2\tfrac{5}{2}(1)-\tfrac12=2 ✓. Note the intercept is negative, which is what tells you a>0a>0.

Answer

1y=ba1x1a,Y=1y, X=1x, m=ba, c=1a\frac{1}{y}=\frac{b}{a}\cdot\frac{1}{x}-\frac{1}{a},\qquad Y=\frac{1}{y},\ X=\frac{1}{x},\ m=\frac{b}{a},\ c=-\frac{1}{a}

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