Find the angle between the planes `x+y+2z=9a d n2x-y+z=15.`

Explanation

If plane are `a1​x+b1​y+c1​z+d1​=0` and `a2​x+b2​y+c2​z+d2​=0` then angle

`cosθ=(a1​a2​+b1​b2​+c1​c2​​)/(sqrt((a1)^2​+(b1)^2​+(c1)^2​)
sqrt((​a2)^2​+(b2)^2​+(c2)^2))​`
​

If `x+y+2z=9` and `2x−y−z=15`

`a1​=1,b1​=1,c1​=2`

and

`a2​=2,b2​=−1,c2​=1`

...