The way you find the angle between two planes is this:
- Look at the line where the two planes intersect
- Find two lines perpendicular to this line of intersection, one in each plane
- Find the angle between these two lines
So, in question 1, the planes intersect along the line BD. This line BD has two important properties, each relating to one of the planes:
- It is the base of the isosceles triangle BGD. (Can you see why it's isosceles?)
- It is the diagonal of the square ABCD.
So, can you see that if we take M as the mid-point of BD, and draw the lines MG and MC, these lines are perpendicular to BD? So it's the angle between these two lines, , that you need to calculate. You can do this by finding the length of MC, and then looking at the triangle MCG.
Do question 2 in a similar way: the planes intersect along the line AC. Mark its mid-point, M, and draw the lines MS and MD.
In question 3, you need to mark in the mid-point of BC, and question 4 is very similar, except that you're given the vertical height of the pyramid instead of the height of one of the slant-edges.
Have another go at these questions. Get back to us if you need more help.