Let pi_1 be the xy-plane and pi_2 be the yz-plane.

(b) Find the angle between pi_1 and pi_2.

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The xy-plane has an equation 0x + 0y + z = 0 and the yz-plane is x + 0y + 0z = 0.

The angle `theta` between two planes is given by `cos theta = (0*1 + 0*0 + 1*0)/(sqrt(1+0+0)*sqrt(0+0+1))`

=> `cos theta = 0`

=> `theta = 90`

**The angle between the plane pi_1 and the plane pi_2 is 90 degrees.**

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