Surya Siddhanta
शङ्कुच्छायाकृतियुतेर् मूलम् कर्णो +अस्य वर्गतः । प्रोज्झ्य शङ्कुकृतिम् मूलम् छाया शङ्कुर् विपर्ययात् ॥
The square-root of the sum of the squares (of the lengths) of the Gnomon and the given shadow is called the hypothenuse of the shadow: from the square of the hypothenuse subtract the square of the Gnomon; the square-root of the remainder will be equal to the shadow; and the length of the Gnomon is to be known (from the shadow) by the inverse calculation.
english translation
zaGkucchAyAkRtiyuter mUlam karNo +asya vargataH । projjhya zaGkukRtim mUlam chAyA zaGkur viparyayAt ॥
hk transliteration by Sanscript