Which Lengths?
Part of the Surface Area & Volume section of Coddy's Geometry journey. Lesson 53 of 60.
A figure can label more than a formula needs. Before you calculate, match the measure to its rule and the rule to its lengths. Volumes use the true height. Surface areas of cones and pyramids use the slant height.
This pyramid asks for its volume, so it needs the base edge 6 and the height 4. The slant height 5 is true but not needed. Had it asked for the surface area, the base edge and the slant height would do, and the height would be the extra.
Spotting the extra length is a skill: exam questions add one on purpose to see whether you know which formula uses what.
Challenge
The cone asks for its surface area. Tap the two lengths the surface area rule uses, then press Check.
Try it yourself
Find the right part on the figure
All lessons in Surface Area & Volume
1Volume of Prisms
Volume of a CuboidDouble Every EdgeVolume of a PrismAny Triangle EndA Missing EdgeRecap: Prism Volume4Pyramids
Volume of a PyramidThe Slant HeightHeight From the SlantHeight From the VolumeRecap: Pyramids8Surface Area of Cylinders
Unrolling a CylinderHeight From the SurfaceRadius From the SurfaceEnough for the Volume?Recap: Cylinder Surfaces11Mixed Solids
Solids From SolidsA Spinning TopWhich Lengths?Letters in the LengthsRecap: Mixed Solids3Cylinders
Volume of a CylinderHeight From the VolumeRadius From the VolumeEnough to Find It?Recap: Cylinders6Spheres and Hemispheres
Volume of a SphereHalf a SphereRadius From the VolumeSphere in a CylinderRecap: Spheres9Pyramids and Cones Outside
Surface of a PyramidSlant Height FirstSurface of a ConeThe Cone: Slant FirstRecap: Pyramid Surfaces