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Example

Maximum Load Supported by a Beam: Direct Variation with the Square

Problem: The maximum load a beam can support varies directly with the square of the diagonal of the beam's cross-section. A beam with a 4-inch diagonal supports a maximum load of 75 pounds. (a) Write the equation relating maximum load to the diagonal. (b) What maximum load can a beam with an 8-inch diagonal support?

Part (a) — Find the equation.

Let LL = maximum load and cc = diagonal of the cross-section. Because LL varies directly with the square of cc, write the direct variation formula:

L=kc2L = kc^2

Substitute the known values L=75L = 75 and c=4c = 4:

75=k(4)275 = k(4)^2

75=16k75 = 16k

Divide both sides by 16 to solve for kk:

k=7516=4.6875k = \frac{75}{16} = 4.6875

Substitute kk back into the formula:

L=4.6875c2L = 4.6875c^2

Part (b) — Find LL when c=8c = 8.

Substitute c=8c = 8 into the equation:

L=4.6875(8)2=4.687564=300L = 4.6875(8)^2 = 4.6875 \cdot 64 = 300

A beam with an 8-inch diagonal can support a maximum load of 300 pounds. Notice that doubling the diagonal from 4 inches to 8 inches quadruples the maximum load from 75 to 300 pounds, which reflects the squared relationship in the variation formula.

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Updated 2026-04-21

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