1.

Show that each of the following pairs of planes are at right angles:(i) 3x + 4y – 5z = 7 and 2x + 6y + 6z + 7 = 0(ii) x – 2y + 4z = 10 and 18x + 17y + 4z = 49

Answer»

(i) Here if θ = 90° we get cos 90° = 0

So we get

A1A2 + B1B2 + C1C2 = 0

By comparing with the standard equation of a plane we get

A1 = 3, B1 = 4, C1 = 5

A2 = 2, B2 = 6, C2 = 6

Consider LHS = A1A2 + B1B2 + C1C2

Substituting the values

= (3 × 2) + (4 × 6) + (-5 × 6)

= 6 + 24 - 30

= 0

= RHS

Therefore, it is proved that the angle between the planes is 90°.

(ii) Here if θ = 90° we get cos 90° = 0

So we get

A1A2 + B1B2 + C1C2 = 0

By comparing with the standard equation of a plane we get

A1 = 1, B1 = -2, C1 = 4

A2 = 18, B2 = 17, C2 = 4

Consider LHS = A1A2 + B1B2 + C1C2

Substituting the values

= (1 × 18) + (-2 × 17) + (4 × 4)

= 18 + (-34) + 16

= 0

= RHS

Therefore, it is proved that the angle between the planes is 90°.



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