CHAM i.u in 



EQUATIONS OF THE FIRST DEGREE 

 Ax + By + Cm + D m 



PLANES AND STRAIGHT LIKES 



I. Tin: I'LAXB 



214. Every equation of the first degree represents a plane. 

 lane in a surface such t ntains every point on a 



straight lim- joining any two of iU point*. 



Let P l s (*,, y r i,) and P f s (x* y v *J be any two i>oint8 

 of the surface whose equ. 



so that 

 and 



Ax+By+Cz* D = 0, 



Ax l + By l 4- CV, 4- D - 



[1G] 



Now, if P 8 =(r r y r *j) be any point mi tin- straight line 

 listance d l from P l and </, from P r then, 

 by A 









. 



(3) 



But this point lies on the surface represented by equation 

 [16]; for, sul >st it u ting ite coordinates from (8) in equation 

 [16], th. Utter becomes 



\X. OEOM. 88 







