# The planes A1x + B1y + C1z + D1 = 0 and A2x + B2y + C2z + D2 = 0 are perpendicular. How to find the value of A1A2 + B1B2 + C1C2?

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`a_1x+b_1y+c_1z+d_1=0`

`a_2x+b_2y+c_2z+d_2=0`

Two plane are perpendicular then

`a_1*a_2+b_1*b_2+c_1*c_2=0`

So substitute the values of a's ,b's and c's in above equation and solve .