Problem 1-35 Determine the largest intensity w of the uniform loading on frame when stress is given 

The cross-section is exactly perpendicular to the QR member. However, the x-x section is not perpendicular to the QR member. Cross-sectional dimensions are given. Therefore, to convert this cross-sectional dimension to that on the x-x section we need to use a right-angled triangle. That's what it is done in this video.