Remember the general rule of transformation:
For c>1, the graph is scaled:
1) y= cf(x) , Stretches the graph of f vertically by the factor of c.
2) y= 1/c f(x) , Compresses the graph of f vertically by the factor of c.
In our case ,
g(x)= cos(x) and transformed function is f(x)= 9cos(x)
So, the rule no 1 is applied, the graph of cos(x) is vertically stretched by the factor of 9.
If you make the table which is
x |cox(x) | 9cos(x)
0 | 1 | 9
2 | 0.999 | 8.99
3 | 0.998 | 8.98
you can see that every value of y is multiplied by 9 in transformed function which will give you a vertical stretch by factor of 9(in transformed function).
B is the correct answer.
