Video: Inside the Tanks - Jagdpanther

TheKroo:   Consider two armour plates: one is vertical, and one has an inclination of 30 degrees from vertical. The armour plates must both offer a protection equivilant to 100 mm thickness. Obviously, the vertical plate must be 100 mm thick. If using pure trigonometry to calculate the required thickness of the other plate, we would get 100 mm × cos (30) ≈ 87 mm. While this result would seem to indicate a weight saving of 13 percent, it does not take into account that the armour plate would have to be longer to cover the same area.
If the armour plate would have to be 1000 mm tall, the height of the vertical armour plate would naturally be 1000 mm. The length of the sloped armour plate, however, would have to be longer, since sloping the armour would reduce the height covered by a plate 1000 mm long. The exact length is easily calculated as 1000 mm ÷ cos(30) ≈ 1155 mm. Since 100 mm × 1000 mm ≈ 87 mm × 1155 mm, it is demonstrated that the weight will be identical of two plates that has the same horizontal thickness and covers the same area, regardless of slope   So technically you are not wrong. But as the Jagdpanther is a tank destroyer, rather than a gun tank, its frontal sloped armour extends from edge of lower glacis panel to the top of the superstructure, thus giving it necessary additional length, and not gaining extra weight compared to what would happen with a gun tank.