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# help with an easy math problem making 'r' the subject?

r^2+r=A/2ph ....make r the subject

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- PinkgreenLv 76 years agoFavorite Answer
r^2+r=A/(2ph)?

If so, then

r^2+r-A/(2ph)=0=>

r=[-1+/-sqrt(1+2A/(ph)]/2

=>

r=[-1+sqrt(1+2A/(ph))]/2

or

r=[-1-sqrt(1+2A/(ph))]/2

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- Ron WLv 76 years ago
Complete the square on r:

r² + r + (1/2)² = A/(2ph) + (1/2)²

(r + (1/2))² = A/(2ph) + 1/4

r + (1/2) = ±√(A/(2ph) + 1/4)

r = -(1/2) ± √(A/(2ph) + 1/4)

This is precisely what you'd get if you wrote the equation as

r² + r - (A/(2ph)) = 0

and then applied the quadratic formula.

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