Teak wood is obtained from which plant?

The number of solutions of the equation $4 \cos 2 \theta \cdot \cos 3 \theta = \sec \theta$,when $0 < \theta < \pi$,is

The length of the common chord of the circles of radii $15$ and $20$,whose centers are $25$ units of distance apart,is

The given graph represents $V-I$ characteristic for a semiconductor device. Which of the following statements is correct?

The energy of a photon is $3 \times 10^{-12} \ erg$. What is its wavelength in $nm$?
$(h = 6.62 \times 10^{-27} \ erg \cdot s; c = 3 \times 10^{10} \ cm/s)$

The focal length of a convex lens will be maximum for: