How should we prove this ? boc=90-1/2angle bac

We have m(<CBO) = (1/2) · m(<CBE) = (1/2) · ( x + z );

In the same way, m(<BCO) = (1/2) ·( x + y);

m(<BOC) = 180 - [(1/2) · ( x + z ) + (1/2) ·( x + y)] = 180 - (1/2)· ( x + x + y + z );

But, x + y + z = 180;

Then, m(<BOC) = 180 - (1/2)·( x + 180 );

Finally, m(<BOC) = 90 - (1/2)·x;

So, m(<BOC) = 90 - (1/2)·m(<BAC).

europa2433

europa2433

step-by-step explanation:

a 4/6 = 9/12   not proportional

step-by-step explanation:

to find which are not proportional, we can use cross products

if it is true, they are proportional.   if it is not true, they are not proportional.

a 4/6 = 9/12

4* 12 = 6*9

48 = 54

not this is not proportional  

b 2/3 = 12/18  

2* 18 = 3 * 12

36 = 36

this is proportional

c 5/4 = 10/8  

5*8 = 4*10

40 = 40

this is proportional

d 7/5 = 35/25

7*25 = 5 * 35

175 = 175

is proportional