If k = 0.1, lambda = 10 angstroms, cos theta= 15, FWHM of the xrd peak is 10 then grain size of the crystal is *1 point0.00660.00550.00440.0061

The question seems to be asking for the grain size of a crystal using the Scherrer Equation, which is used in X-ray diffraction (XRD) to determine the size of crystals in a solid. The formula is:

D = Kλ / (β cos θ)

where: D is the crystal size, K is a dimensionless shape factor, with a value close to unity. The shape factor has a typical value of about 0.9, but varies with the actual shape of the crystallite, λ is the X-ray wavelength, β is the full width at half maximum (FWHM) of the X-ray peak, θ is the Bragg angle.

However, there seems to be a mistake in the question. The value of cos θ cannot be 15 as the range of the cosine function is from -1 to 1. Please check the values and try again.