# Ernesto's question at Yahoo! Answers regarding harmonic and arithmetic progressions

#### MarkFL

Staff member
Here is the question:

Prove that if b+c, a+c, a+b are in harmonic progression, then a², b² y c²...?

Prove that if b+c, a+c, a+b are in harmonic progression, then a², b² y c² are in arithmetic progression .

Explain.

Thanks.
I have posted a link there to this thread so the OP can view my work.

#### MarkFL

Staff member
Hello Ernesto,

If $b+c,\,a+c,\,a+b$ are in harmonic progression, then their reciprocals are in arithmetic progression, and we may state:

(i) $$\displaystyle \frac{1}{a+c}=\frac{1}{b+c}+d$$

Multiply through by $$\displaystyle (a+c)(b+c)$$ to obtain:

$$\displaystyle b+c=a+c+d(a+c)(b+c)$$

Subtract $c$ from both sides and expand the product on the far right:

$$\displaystyle b=a+d(ab+ac+bc+c^2)$$

Distribute on the right:

$$\displaystyle b=a+abd+acd+bcd+c^2d$$

Subtract through by $$\displaystyle a+abd+acd+bcd$$:

$$\displaystyle b-a-abd-acd-bcd=c^2d$$

Divide through by $d$ and arrange as:

$$\displaystyle c^2=\frac{b-a-abd-acd-bcd}{d}$$

(ii) $$\displaystyle \frac{1}{a+b}=\frac{1}{b+c}+2d$$

Multiply through by $$\displaystyle (a+b)(b+c)$$ to obtain:

$$\displaystyle b+c=a+b+2d(a+b)(b+c)$$

Subtract $b$ from both sides and expand the product on the far right:

$$\displaystyle c=a+2d(ab+ac+b^2+bc)$$

Distribute on the right:

$$\displaystyle c=a+2abd+2acd+2b^2d+2bcd$$

Subtract through by $$\displaystyle a+2abd+2acd+2bcd$$:

$$\displaystyle c-a-2abd-2acd-2bcd=2b^2d$$

Divide through by $2d$ and arrange as:

$$\displaystyle b^2=\frac{c-a-2abd-2acd-2bcd}{2d}$$

(iii) $$\displaystyle \frac{1}{a+b}=\frac{1}{a+c}+d$$

Multiply through by $$\displaystyle (a+b)(a+c)$$ to obtain:

$$\displaystyle a+c=a+b+d(a+b)(a+c)$$

Subtract $a$ from both sides and expand the product on the far right:

$$\displaystyle c=b+d(a^2+ac+ab+bc)$$

Distribute on the right:

$$\displaystyle c=b+a^2d+acd+abd+bcd$$

Subtract through by $$\displaystyle b+acd+abd+bcd$$:

$$\displaystyle c-b-acd-abd-bcd=a^2d$$

Divide through by $d$ and arrange as:

$$\displaystyle a^2=\frac{c-b-acd-abd-bcd}{d}$$

Now in order for $a^2,\,b^2,\,c^2$ to be in arithmetic progression, we require:

$$\displaystyle b^2-a^2=c^2-b^2$$

Add through by $$\displaystyle a^2+b^2$$:

$$\displaystyle 2b^2=a^2+c^2$$

Substitute for $a^2,\,b^2,\,c^2$ the expressions we found above:

$$\displaystyle 2\left(\frac{c-a-2abd-2acd-2bcd}{2d} \right)=\frac{c-b-acd-abd-bcd}{d}+\frac{b-a-abd-acd-bcd}{d}$$

Distributing the 2 on the left, and the multiplying through by $d$, we find:

$$\displaystyle c-a-2abd-2acd-2bcd=c-b-acd-abd-bcd+b-a-abd-acd-bcd$$

Add through by $$\displaystyle 2abd+2acd+2bcd$$:

$$\displaystyle c-a=c-b+b-a$$

Collect like terms:

$$\displaystyle c-a=c-a$$

Subtract through by $$\displaystyle c-a$$:

$$\displaystyle 0=0$$

This is an identity, which proves that given $b+c,\,a+c,\,a+b$ are in harmonic progression, then $a^2,\,b^2,\,c^2$ must be in arithmetic progression.