Engineering Mechanics - General Principles - Discussion

Why cant we use cos40=7/AB=>AB=9.14 directly......it is also correct method.

Need to apply sin rule,
AB/sin(C) = BC/sin(A) = CA/sin(B)

IN THE PROBLEM,
AC=7, BC=6 AND ANGLE A=40 GIVEN

AB/sin(C) = 6/sin(40) = 7/sin(B).
sin(B) = [7*sin(40)]/6 from this we will get B angle.

Then C=180-A-B.
AFTER THAT AB = [6/sin(40)]*sin(C).

I think 1st we check about diagonal ,so according to pythagorean triangle formula. 6*2+7*2=h*2( * means power ) so h =9.22 , but if angle angle acb is 90. So then see the option, in option A and option D h=9.22, but in that 2 option angle acb not 90 degree.

Then in option C angle is 90 degree but diagonal h is not 9.22, so this is also incorrect. Because if angle is 90 then h must be 9.22.

So easily you without calculation you may find option D is. correct.... thanks.

bc = 6, ac = 7 ab = ? now, trigonometric from ab^2 = bc^2+ac^2.

ab^2 = 36+49 = 9.21.

Answer = D.

We can not use trigonometry or Pythagoras theorem unless we have right angle triangle, here the figure is seems to be right angle triangle but it is not.

So we can use sin formula a/sinA=b/sinB=c/sinC where a, b, c are the sides opposites to the angle A, B, C.

I totally agree with you @Devendra, by using sin formula one can solve this problem in an accurate manner.

In triangle ABC, angle A is given 40, By applying the formula.

6/sin 40 = 7/sin B, we get sin B = 48.58.

Now the sum of all angles of triangle = 180.

We get angle C = 91.4.

Again 6/sin 40 = c/sin 91. 4, we get C = 9.33.

Guys here we can't use trigonometry relations.

Because one of them angle is not 90°.