Mathematical Methods for Physics and Engineering : A Comprehensive Guide

(Darren Dugan) #1

PRELIMINARY ALGEBRA


drawn throughR, the point (0,sin(A+B)) in theOxysystem. That all the angles


marked with the symbol•are equal toAfollows from the simple geometry of


right-angled triangles and crossing lines.


We now determine the coordinates ofP in terms of lengths in the figure,

expressing those lengths in terms of both sets of coordinates:


(i) cosB=x′=TN+NP=MR+NP

=ORsinA+RPcosA= sin(A+B)sinA+cos(A+B)cosA;

(ii) sinB=y′=OM−TM=OM−NR
=ORcosA−RPsinA= sin(A+B)cosA−cos(A+B)sinA.

Now, if equation (i) is multiplied by sinAand added to equation (ii) multiplied


by cosA, the result is


sinAcosB+cosAsinB= sin(A+B)(sin^2 A+cos^2 A)=sin(A+B).

Similarly, if equation (ii) is multiplied by sinAand subtracted from equation (i)


multiplied by cosA, the result is


cosAcosB−sinAsinB=cos(A+B)(cos^2 A+sin^2 A)=cos(A+B).

Corresponding graphically based results can be derived for the sines and cosines


of the difference of two angles; however, they are more easily obtained by setting


Bto−Bin the previous results and remembering that sinBbecomes−sinB


whilst cosBis unchanged. The four results may be summarised by


sin(A±B)=sinAcosB±cosAsinB (1.18)

cos(A±B)=cosAcosB∓sinAsinB. (1.19)

Standard results can be deduced from these by setting one of the two angles

equal toπor toπ/2:


sin(π−θ)=sinθ, cos(π−θ)=−cosθ, (1.20)
sin

( 1
2 π−θ

)
=cosθ, cos

( 1
2 π−θ

)
=sinθ, (1.21)

From these basic results many more can be derived. An immediate deduction,

obtained by taking the ratio of the two equations (1.18) and (1.19) and then


dividing both the numerator and denominator of this ratio by cosAcosB,is


tan(A±B)=

tanA±tanB
1 ∓tanAtanB

. (1.22)


One application of this result is a test for whether two lines on a graph

are orthogonal (perpendicular); more generally, it determines the angle between


them. The standard notation for a straight-line graph isy=mx+c,inwhichm


is the slope of the graph andcis its intercept on they-axis. It should be noted


that the slopemis also the tangent of the angle the line makes with thex-axis.

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