Which previous hypotenuse is used to construct √50 in a square root spiral?

Correct answer: √49

The governing idea is the repeated use of the Pythagorean theorem in the square root spiral. At each stage, a perpendicular segment of length 1 unit is added to the preceding hypotenuse. If the preceding hypotenuse is √n, the new hypotenuse h satisfies h²=(√n)²+1²=n+1. To construct √50, we need n+1=50, so n=49. Hence the immediately previous hypotenuse is √49, and option B is correct. √48 is two stages earlier in the sequence. √50 is the target length being constructed, not its predecessor. √51 would occur at the following stage after √50. The relationship between consecutive squared lengths therefore identifies √49 uniquely as the required previous hypotenuse.